A good charger will stop actively charing the batteries once they're complete, so in general it should be safe to do so. For myself, I keep the charged spares in my camera bag, along with the charger, and alternate as I go. My spares aren't going to be a lot of use if they're stuck to the wall and I'm out in the woods with my camera. :)

It definitely depends on the charger as John has already said.

I've got a decent charger, even so I charge the batteries and then remove them from a charger. The batteries are then kept in a charged or flat tray. Though I do have Imedion / Eneloop batteries so they hold the charge longer than most rechargeable AA's.

I pretty well always have a 4 or 8 AA Eneloop batteries on my Maha charger, and I always have a Canon camera battery on its charger as well. I have done this for years and have never had an issue. Good Li-Ion batteries and good chargers will allow you to do this without any trouble.

Note that John's point about keeping them in the bag rather than on the charger is a good one, but I own many sets of batteries and several batteries for my cameras as well, so I also always have spares in my camera bag too.

Modern camera batteries that use Lithium-Ion as their main technology have specific requirements on charging, so their chargers have always needed to be "smart" enough about what to do with a fully charged battery. In some applications, the batteries themselves even contain circuitry that make them "smart"; they can remember their profiles, even provide a 'gauge' type display using LEDs. Your laptop computer's battery most likely has one of these.

Older technologies, like NiCd and NiMH didn't require such measures, so 'smart' chargers are not commonplace. There's no inherent danger to overcharging, since the batteries will simply dissipate the extra current as heat and they will warm up, but not explode. Overcharging such batteries do however, has a damaging effect in the long run.

That's why smart chargers exist for these batteries as well, but they're hard to spot. The charger that I use now, the Voltcraft IPC-1L for example, will determine whether the battery is fully charged and switch to trickle charging to keep the battery in the best condition possible until I need it.

I think the chargers that you should not use to keep your batteries in and plugged are the popular "fast chargers". I would steer clear from them anyway, since their high current charging technique might charge your batteries faster, but they will never be as fully charged as they can. Moreover, most of these contain only a simple thermal safety sensor that will stop the charging when things get too hot, not when the battery is done charging.

I've seen batteries develop 'blisters' in the plastic labeling on them, by leaving them in such a charger. I can only imagine what the condition of the chemicals must be.