If you charge the phone overnight, turning off your Internet connection (WiFi or 2G/3G) and switching to GSM from 3G will make the loss of battery charge negligible. Remember, your N900 can go almost for two days on total standby so a few hours doesn't mean much. The CPU downclocks itself a lot, the screen is off. Well, at least, that's my experience.

And if you use (provided it works with your N900) the ulv kernel the loss is even smaller as it will give you lower voltage which uses even less power at minimum clock speeds.