I assume you mean purple salvia of some type. If so, I would wait until you see new growth coming up around the base of the plant before cutting it back. (If that has already happened, go for it.)

What I do is break some off and see if there is any green.

I trim my salvia guarnetica by about 1/2 and then wait for the ground to warm up to see what else needs to be trimmed. But I do that because I already know how it grows.

But I'd imagine different salvias don't die all the way back so if you are unsure, then wait.
Q