Nah, growling's a big part of the game! Mine sounds like angry grizzly bears when they play.
Now if he gets a real hard stare, bares teeth, raises hackles, etc...that MAY mean he's taking it a wee bit too serious. You can pretty much read his body language. I bet that's not the case though.
In the event someone has a dog who is getting real aggressive about these sorts of games, IMHO it doesn't do any good at all to just stop playing it. Better to teach him some game ground rules.
