The only problem I've ever had with prekilling them and just leaving them in my snake's cage is : if the mouse gets too cold (after being dead for a while) your snake may not be interested in it. I prekill my mice and basically dangle them in the snake cage until my girl strikes. (usually doesn't take long, but they will be more attracted to the movement) If you DO leave it in her cage, I'd maybe recommend leaving it where it might stay warm (thanks to a uth), but not hot enough to cook it (snakes can't digest cooked food/meat).

Hope that helps.

Btw: I use the "blunt trama" method. One good blow to the head/neck so there's no pain (or suffocation like with CO2). Just some advice for that though-don't hesitate. If you do, it will take more than one hit and WILL cause the mice/rats pain, and I think that part would be cruel.