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.