Reserving the string to the proper fit is the BEST solution. You can bend and sand nocks for a better fit, but not with the consistency you would want. then your string fits a couple "modified nocks" but not a standard size.
If you try someone else's arrows, be CAREFUL.
Second best solution is to fray unwaxed dental floss over a knife blade and wrap with Duco cement around the nock point. You can reapply Duco when floss starts to wear. This can last a long time and will give you a safe and secure fit. I think Viper said a nock has to be really tight, REALLY tight, to give you accuracy problems.
Nocks that are too loose are an accident just waiting to happen and an ounce of prevention is worth 45-65# of cure (as in a new bow, from a dry-fire).
My two cents.
Doug