The great debate of the century...
I keep mine in sleep mode all the time. I haven't turned it off for at least a week or so. Start-up times are so long, and sleep mode doesn't eat that much batter. Running a 16GB with 1.1.3 & Software Update (not Jailbroken). I did notice with some of the 3rd party apps, they consumed a lot of battery in sleep mode. Hope this helps.