Depends what kind they are. Generally I think most manufacturers recommend you wait until the batteries are dead (or near enough to) to maximise their life span, but I have had some that advertise the fact that you can recharge them whenever you like. If you're not sure, check the packaging (if you've thrown it out, find the same type at a shop perhaps)
If they are the old Nickel Cadium type (hopefully not, since these are way old technology) they are very picky about charging cycles. That being waiting until the camera thinks they are dead and then charging them for an uninterrupted amount of time. If you don't do this with (NiCad's for short) they can develop "battery memory" and whenever you plug it in at say, 3/4 charge to top-it-off they will think they are dead at that time if you do this frequently. For many years now though, most rechargeable batteries are the Nickel-Metal type (NiMH for short) and they do not suffer from this. You can pretty much do whatever you want to them in terms of charging and discharging.
"A piece of toast with butter always lands butter side down, and a cat always lands on its feet. What happens if a piece of toast is tied butter side down to the back of a cat? Does it perpetually hover above the ground in indecision when dropped?"
My experience with Li-ion batteries in a laptop is that they'll take just about anything (except being frozen). When you do start to notice the capacity dropping, you can deep-cycle the battery, that is, letting it discharge as much as possible before recharging fully. This will usually improve the capacity of the cell back toward the original specification.
Thanks.