If you fly on Southwest, often as you board the plane they announce that the bins are full in the back and you should find a bin on your way back. Works for me, I just follow instructions.

On other flights, I do find it rude to take someone elses bin space, but if the flight has finished boarding, what is wrong with finding open bin space somewhere else to make room for my legs and feet? Is it my fault I am 6'3" and DL keeps adding rows instead of removing them?