Is it OK to put casters on the bottom of speakers?

I think it's a very good idea. Others will surely disagree. If I were going to do it, I would attach the castors to a separate base that is actually wider than the speakers for stability. Then I would simply screw the base to the bottom of the cabinets which should do minimal damage.

At the risk of castering aspersions please don't do this to your speakers.

Evan,
I have a similar problem whereby my large speakers (Avantgarde Duos) have to be moved forward from the back wall every time I listen to music. The solution I settled on was furniture "Sliders" I placed under my speakers. They are 1/2 thick and round, come in various sizes and have a sticky side that is placed against the bottom of the piece of furniture. They are teflon coated and easily glide over all surfaces including carpet. I bought them at Home Depot-- very cheap. No drilling into the speaker or otherwise permanent damage is necesary. Enjoy listening.

Oh dear, you have a dilemma. Let us know how you solve it. The tradeoff between mobility and rigidity is a terrible one. Ideally you want to anchor your speakers as firmly as you can, while maintaining a high impdedance mismatch between them and the floor. Cones and spikes do this quite well but casters, even locked, don't. Totem speakers use a ball bearing foot ( called a "claw" ) which sounds better than IMHO it should. Maybe something like that would be a good compromise.