Hmm I wouldn't. But if that battery dies it can't be used while charging. Though 2 minutes of charge should be enough to use it.
To be fair, I always turned my Magic Mouse off and on. I've never done this with my keyboard and Magic Trackpad...though they don't have an on/off button to do so.
It just seems annoying to turn 2/3 devices off and on every time. Your bound to forget...so I would just keep them on all of the time unless your noticing severe battery drain.
The devices should be smart enough to hibernate when not in use but I don't have enough experience with the new ones to know 100%