Anything from 50 pf - 500pf is usually used.
Silver mica caps are useful, because they tend to pick up less filament AC Buzz.
Volume control with 40-60% tap is sometimes used, to smooth out the curve in frequency response.
Usually the best result is obtained by experimenting w/ different caps, playing guitar, and adjusting it to taste.
A lot of more modern amps are using 50 pf, as opposed to the older style 250-500.
Using 50, the highs are smoother, and the effect is less harsh, in many amps...it does sound better.