I think running over can be helpful if it's needed. I prefer T give me the extra time if she has it to give and if I'm in the middle of something, but it also makes me super-anxious to go over by much (5 or 10 minutes is ok, but 20 or 30 sends me into a panic).
If I know we are going ot be covering something really stressful or difficult, I try to ask for a longer scheduled session before hand so I don't feel guilty.
I tend to see keeping time commitments as a respect thing (at least in professional transactions). If a T regularly goes over time, I would just rather them to schedule longer sessions so the next person isn't waiting when they could be doing other things. I feel the same way about my doctors, or any service provider. If it's an occasional thing, it's no big deal.

Oh, and if you feel like you get into things late in the session, maybe you could talk to T about that. I find I struggle with that a lot, so much of what I need to process isn't complete by the time session ends. When that happens, I end up ruminating over it and unable to box it back up successfully between sessions. Definitely something to work out with T (I have recently switched T's way more frequently than I'd prefer, so it's a constant conversation).