Personal Note on the application of the spherical tip on commercial LP records:

I read most significant passage in NAB(1964) as follows:
"In disc recording, it is the generally accepted practice to evaluate sound quality and musical balance of a disc on a reproducer which has a specified response - frequency characteristic".
IMO: True Fidelity based on historical background is different from so-called Hi-Fidelity using most advanced equipment.
Often I hear unbalance (emphasis on high frequency band) at inner radius of record with using modern line contact styli on old records. Percy Wilson commented in his book "THE GRAMOPHONE HANDBOOK" (P.42-43: year 1957) about the reducing line speed toward inner radius of record: "This causes a progressive attenuation of treble notes in reproduction from the outside to the inside of a record. To compensate for this some recording companies use a progressively increasing treble pre-emphasis as the stylus moves across the record. This is known as "radius correction"." In my estimation many recording companies since SP era had applied radius correction plus standard recording curve during recording process in order to make good sound balance across the records. When old records have been recorded in such way (pre-compensated suitably for reproduction by spherical tip), then advanced line contact stylus with reduced tracing loss would reproduce unbalance (emphasis on high frequency band) at inner groove radii of records.  It is quite questionable whether modern records have been recorded or designed suitably for any special styli or not.
Hence I would prefer to use conventional spherical tip radius 0.6-0.7mil (15-18microns) for stereophonic and monaural LP records.
        Some requirements in standards are changing by time. 
        In 1963 RIAA recommended to use spherical tip radius 0.5 mil (13microns) as desirable for stereophonic groove,
        In 1964 IEC recommended to use spherical tip radius 13-18 microns for stereophonic groove,
        In 1972 by amendment slip to BS1928 and 1975 by DIN45500 and in 1978 by IEC60581,
        spherical tip 15-18microns was recommended for both mono & stereo application.
        Note that LP production in most countries (USA/Japan/UK) reached the highest around 1975. 
        I don't know the reason (*) why the previous tip radius 13microns (0.5mil) was omitted from these recent standards.
Sometimes recent standards revert to the older standards  because of finding certain inconvenience or unreality of the assumed advanced one.  I find similar example in the description of maximum groove bottom radius in μm: 7.5(1958IEC), 6.35(1963RIAA), 4(1964IEC), 8(1972BS/1981DIN/1987IEC).

(*) I assume some reasons why the spherical tip radius 13microns (0.5mil) was omitted since 1972, around when discrete 4 channel  (CD4) records appeared.

1. From the studies and experiments of CD4 era: Light VTF and large area of stylus contact were required for tracing the supersonic modulated groove. 

2. When Static Vertical Tracking Force is fixed around 2 g, contact area of various tips in ascending order: spherical 13micron(0.5mil) eqv. to elliptic stylus < spherical 18micron (0.7mil) eqv. to Shibata & other line-contact styli - see details in stylus.xls including Walton's report(1961). Contact area and wear were focused in that time so that stylus for conventional monaural & stereophonic use was also refurbished as described in 1972 by amendment slip to BS1928-1965.  In other words: the tracing/fundamental loss at high frequencies due to the dimension of stylus was focused at first as in case of 1963 RIAA recommendation, but in the next era (early 1970s) translation loss due to record elasticity and stylus pressure was also taken into consideration. Small radius stylii reduce tracing loss and pinch effect while small stylii with small contact areas increase translation loss at high frequencies and also make high frequency resonance lower than 20kHz: Vinylite records have variable elasticity depending on material, temperature and also groove undulations. We look one side only and overlook other. One time experiment or measurement is not reliable because different conditions make different results.

3. Japanese patent paper 1984-33603 by Denon commented: "Up to now, smaller tip radius had been recommended in order to comply with high performance of record. Meanwhile its small contact area reduces the effective stiffness of record and shifts the high frequency resonance lower than 20kHz and also digs the groove wall deeply. In order to solve such problems, line contact stylus with larger contact area was applied. But the manufacturing process of such stylus is costly and complicated. Moreover after some time of use, the contact area of line contact stylus is increased by tip wear so that the life span of such stylus is relatively shorter."  IMO:  Micro Ridge （Shure V15VxMR 0.15x3mil）/Micro Linear (AT) etc seems to be based on USP 4521877(1985) by Namiki Precision & Jewel where the diamond ridge can be worn by time of use, but the effective radius of contact (=ridge width/2) remains unchanged so that its life span for use without deteriorating the characteristic is prolonged over 1000 hours of use.  This Denon paper pointed out the high frequency resonance (between record elasticity and effective tip mass) around 20kHz - but many popular cartridges showed such peaks lower than 20kHz actually and most users are satisfied because they enjoy such as one of the characteristics of these cartridges. What's the problem? It looks simple: users would like to take their meals in their own dishes (something peculiar to them).

Considerations on the inclination of cutter needle and V groove.
1    F.L. Capps in his US patent 2187512-1940 (recording stylus) commented: "during a recording operation
     the cutting face of the stylus is at an angle or 90 degrees to the surface of the blank being engraved or cut,
    although the 90degree angle may be varied approximately 5 degrees forward or backward."
    IEC & BS specified Groove Angle 90 degrees plus minus 5 degrees.
    Their standards are practical when considering
    I. the back-and-forth inclination of cutting stylus as mentioned by Capps 
        This change of groove angle is negligible: 90.22degrees instead of nominal 90degrees even when stylus inclined 5 degrees.  
            Equation for Formed Groove Angle    =    2*DEGREES(ATAN(1/(COS(D*PI()/180)))) 
            Condition: Nominal Groove Angle 90 degrees        D: inclination angle in degree   
    II. the deformation of groove at recording master disk and pressing (heat and cool) process
2    Actually in addition to back-and-forth of cutting stylus
    V groove itself is little inclined sidewise too due to the construction of cutter heads during modulations.
3    Under the above circumstances, the maximum curvature of "special" reproducing styli is kept not larger than 100 micron. Perfect fit between stylus and groove is not preferable, rather loose fit is required for smooth reproduction of records. Nevertheless 0.5mil (13micron) spherical tips are found more popular in market than other size of spherical tips maybe because nominal hi-fidelity is appealing to some snobs who are evaluating value by number or price or brand.