I'd still ask explicit permission. I don't think (since you asked for our opinions on the ethics) that you automatically have permission to make copies of a piece of art simply by buying it, even by buying the original. The artist may well be flattered, of course.

(IANAL again: And legally there are very few cases where it's true that you can make your own copies of a piece of copyrighted art-- it's even been batted up and down for a while whether it's legal to rip your CDs, though I think that one's been settled that you can. In that case, of course, you're making a copy that nobody but you will see anyway. One of the explicit exceptions in federal law is that you can take a photo of a building without infringing the copyright on the architect's design!)