Uni play growls also. When she was a pup my bf would make her growl while playing, and his friend said not to do that b/c it will make them aggressive. I don't think that growling while playing is what makes a pup aggressive, I think it's fear. I think like a human bully, they try to show their toughness b/c they are actually afraid of confrontation. That's just what I think though.
Anyway, Uni is not aggressive to people or dogs, but she growls a lot when playing. I think they way you can tell the difference is if they show teeth/gums when they are aggressive. When she growls during play, she usually has a toy in her mouth.
