The guitar has Millivoltage running through the circuits. So in essence it doesn't really matter. The concern becomes size of the caps. Lower voltage caps usually are smaller in size also and easier to manipulate in the cavity.
I ordered .022uF + -5% 50v DC polyester #dsf050j223 from Newark.... part #98b5764
and .330pF + -5% 50v DC #mcc1c331j1hs5L-RH from Newark.... part #80k9091
Ckeck Mouser electronics: http://www.mouser.com/
or Newark Electronics: http://www.newark.com/
Try these too..... Vishay is a good brand.