Trust me, you can sit anywhere. It may not be the proper rule but everyone does it: Don't move until you're asked by someone with ticket proof. There have been many a fiesty old lady or three that think you don't know what you're doing because you're white, when they're doing the same thing you are.

At the most, the conductor goes through and has his seat chart. I've seen him tell others that they can sit there until XXX stop when someone has reserved the seat. Nothing serious happens.