If they are used infrequently then it's best to remove the battery I think - this will also keep the battery from running down. If the battery is dead then just throw it away and run the Mac without it - no battery is better than a dead one as dead ones just leak and cause damage. I think it's better for the capacitors if the machine is started up every few months, though there's not much you can do to stop them leaking, this sadly happens in the older machines and has lead to a lot of Mac deaths ( :'( ) - the machine will work with leaky capacitors until there is so much gunge that it blocks circuits on the board, you can then clean it off to get it working again but I tried wiping it off in my LC II and it still doesn't work, I think you would have to risk the "immerse in water" thing to get it off and returned to a working state.