I've had to travel with a large poster tube many times. Never an issue. I usually board early, pop it right into the back of the overhead bin and it doesn't really take up any usable space (since the bins are angled at the back a full-size wheelie case still fits in front of it).
I concur -(very much against the advice provided by C-W-S) - much better to put the tube in first. Done it many times even with A0 size although it was a few years ago.