Only if I'm at one of these very very rare old tables that is made of solid wood. We had one at work (we called the room "Hogwarts" as it was full of antique solid wood furniture), I think it's gone now.

I do use a beer mat in the pub if I can help it, because it's really annoying when condensation on the outside of the glass runs down and you end up with your glass in a small pool of water (extra annoying if you're trying to read a paper or use a quiz sheet or whatever if it gets wet). Bugs the smurf out of me when pubs don't provide them. Wetherspoons, I'm looking at you.