People will have different opinions on this. When I got my 3GS, I simply let it charge while I synced with iTunes and then went about using it thru my day. Came back and charged again when the level was low.
Just make sure you calibrate your battery in the first few days (drain the battery all the way to zero until the phone shuts down and then charge it for a good 4 hours until it reaches 100%). It will help you get an accurate battery level percentage indicator.
More information on batteries here -
http://www.apple.com/batteries/iphone.html
Message was edited by: mechie8682