I have more than one sets. I keep a set in the camera and a set in the charger. After two weeks, the batteries in the camera tends to go 'dead' if I don't use the camera. With frequent use of the camera, I noticed the batteries last just a couple days. So a set left on a continuous charge works for me. Hope this shed some light for you.
Some chargers will recharge the batteries, then go into top-up charge mode. With these it's safe to leave the batteries constantly on charge. However, these are usually the more expensive microprocessor-controlled chargers. My favourite for a long time has been this one.
With a cheap charger, it'll keep trying to charge an already-charged battery at a high rate, with possible results Weeal mentions.
The charger manual will always tell you whether or not to remove fully charged batteries
Although at a slight premium,a number of companies now supply NiMH batteries with a much better self discharge performance,so they retain their charge much better.Examples are Uniross Hybroi,Sanyo eneloop and Ansmann MaxE The latter are from Tantronics.When you next need to buy replacements I can recommend these.
first point ... if you can ... don't fully discharge !
not much of an issue with LiLon but all the others prefer to have a little left in reserve ... (it's a chemistry thing)
when you store them ... best not to fill em right up if you intend to leave them more than a couple of days without use.... the optimum is 25% .... that's why when you buy new they always have a bit in em