OK for Lithium based batteries leaving it charged often is better for the health of the battery BUT there is a catch. you also need to exercise the battery every now and then (not sure why they degrade if they are always kept at 100%) this makes sense since its bad for them to be stored at 100% (40% is the recommended storage charge) so I would let 2 bar go away and then recharge whenever convenient.
As for exercise this is my plan. every six months or so I plan to run some mp3's and kill it and then recharge and kill it again. That should keep the cells in shape.
Since I started doing this with my laptops I have stopped killing battery packs. :-)
(my laptop is a desktop replacement so it only runs 35-45 minutes on the battery so its ALWAYS plugged in and I kill the battery once a year (as in dead killed have to replace it) since I started exercising it more often I get several years out of them now.
Chris Taylor
http://www.nerys.com/