The only one I put a backing on was one that I made for my husband at work; I put sticky-back felt on the back. I HAVE a roll of cork around here, but I can't find it... when I do I'll put it on the backs of the coasters.
Here it is, if you want to see it.
Apologies for the photo quality - his phone doesn't take very nice close shots.
I've got a few others too, that I've been meaning to post.