Maybe it's because it's what my parents did, but if I put a cup/glass down on wood I use a coaster. If I'm at someone's house and I can't see any, I would ask for one (and only put it down if they say it's fine/what they usually do).
I have solid wood fireplace/ table/ window ledges etc and despite having coasters on all of them, I find a few people just ignore them and stick glasses/cups on the wood. This really annoys me as you end up with hard/impossible to remove watermarks. That said, I want guests to feel comfortable and not awkward so don't want to seem rude picking their cup up and moving it onto a coaster. I try to do it subtly but am I being unreasonable to think if there are coasters there you should use them?