My wife and I flew to Europe this past spring with a poster tube, and didn't have any problems. (Well, one local carrier insisted we check the tube as it looked to be oversized, but only one-way. I suspect we had a communication problem due to a language barrier.)

As always, check your airline's rules on carry-on items. Some have a limit on total linear size (length + width + height) of a carry-on which is quite small; your poster tube might be long enough to run afoul of that limit.