I almost always do. It seems like the appropriate thing to do, especially in the U.S. where the staff is not as doting in general (and I don't mean that as an insult).

It seems, from my typical experience in lounges, that I am part of a small minority. Although from this thread, it seems like I am part of the majority. I suppose posters here are not representative of all Marriott (even CL) customers!