I just finished my first century ride (LA River ride) and had a question on etiquette. Is it OK to stick behind someone and draft them for a while?

Bonus question: I think I'm starting to pick up the hand signals. One I wasn't sure about was a first behind the back with a finger up, it was the index finger... I assumed this meant single file since we were all in the street between the parked cars and the moving cars.

Adam