When I am finished with my iMac for the day, I put it in sleep mode, leaving the mouse and keyboard turned on. I use the mouse and keyboard heavily and after a month, the battery level on both is still showing 100%, so I don't think that leaving them on when the computer is sleeping will cause significant battery drain. I always use sleep mode and I don't think I have actually turned the computer off in the month I have had it, other than restarts when doing updates, etc.
I find my mouse battery drains a lot quicker than the keyboard. I just changed the mouse battery's for the first time since I got it in December but the keyboard still says 100% so I usually just shut my mouse off and let the keyboard sit on.
For the keyboard, I wouldn't worry about it. For the mouse, I often turn it off before putting the computer to sleep (since I always invoke sleep via the keyboard), which seems to help the battery life of the mouse.