If you board in later groups (i.e. not one of the first 100 or so people) you are essentially stuck waiting in a line in a cramped isle for people to put their bags up and are likely to be relegated to a middle seat.
Though that does presume being ready to board at precisely that moment while traveling with small children. In my experience I can expect that to happen about half the time.
If you are clearly a family, and miss family boarding, I believe they'll let you board whenever you're ready. As long as you get on sometime in the "B" group, you should find an empty row or two at the back of the plane.
