I'm sure I'm in the minority, but I believe in "first come first served" frankly. As long as you meet the carrier's size requirements, you get on first, you get the space. Now frankly I feel you should have some concern / compassion / politeness to use the space below in order to make it easier for the late boarding folks, but frankly you somehow managed to get early boarding so it's your choice whether I agree with it or not. For late boarding people, they will always have the space below for their personal items and they need to understand if it's full, they it's getting checked. Plenty of easy ways to get early (or "earlier") boarding like a credit card. Again, I would hope people are polite enough to try to use the space below and save overhead space, but if I'm late to boarding or late in the zones, it's my personal responsibility to deal with it.