In my experience, the overhead bins are usually not full until you get to maybe the second half of the LAST boarding group.
Of course it all depends on the plane and size of the luggage people are putting up there.
It's really annoying when people put BOTH their carry on and personal item (briefcase, purse etc) in the overhead bin. These people are effectively taking up 1.5-2 spaces in the overhead bin therefore forcing more people to have to check in their carry ons. There are also people who put their huge overcoats up in the overhead bins even when the FAs specifically say not to.
I mean, can't we all be more considerate?