It depends on the guitar and your personal tastes. 250k pots are what most people use but don't be afraid to try 500k pots if you are installing the Classic Stack with humbuckers or if you find the guitar is too dark sounding with 250K pots.
if the pickups are the classic stack plus then i would go with 250k, thats what i use and i am very happy with the tone. if they are the old classic stacks then 500k might be better if your guitar isnt naturally bright or if you like a brighter tone as they can be a bit dark in some guitars.
changing the value of the tone control will have a more subtle effect than changing the volume control