Allow a little margin over peak AC value should be OK.
Max.

 

The one I'm making is based on a triac. I figured it would be more fun to build one than to buy one. So, regarding the caps on the circuit, I need to use ones that are rated for the peak voltage of the mains, correct?

An old rule of thumb was, cap volt rating, at least one and a half times the highest circuit voltage. Higher rating is better than lower.