In general, I don't like being late to things-- I'm much more likely to be the person sitting there ten minutes early for some reason. Today, however, one thing led to another and I found myself running late for much of my schedule.
At what point when late is it best to just not show up at all? Nobody would look twice at someone arriving in class twenty seconds late, but thirty minutes in, most people would say it's not worth showing up at all. My personal scale is based on the importance of the event and the publicity of the entrance. Ten minutes late to an one-time-only talk? Go for it, especially if you can just slip in the back. Ten minutes late to a concert with only one entrance near the stage? Maybe another day.