I'm going to start pre-killed with my balls, but right now I just have two milksnakes that are reeeaaaally young so they're just eating pinkies. Once they get up there though, I'm either going to build to CO2 (which is definitely the most humane) or whack 'em, which I'm not looking forward to!

Mice are generally pretty okay, it's really rats that are the big worry. :/ But! It's always good to ere on the side of caution, so pre-killing is a good idea if your snake will take it. There's ways other than whacking, like separating the spinal cord in a few different ways... but rats are squirmy, and I'd imagine it'd be easiest to whack. :/

If you don't want to do pre-killed, it's perfectly fine to do frozen. :]