LOL, and here I am, working so hard to get my dog to growl when he plays tug. Because a growl is a sign he's really into it and I want it to be a strong reward and I want him to be into it.
I agree, a growl is not always a threat. My dog growls when he plays. And once in a blue moon while he's dreaming.
I also strongly agree that you do NOT stop the growl. Instead you find out what is causing the growl and work to fix that, if it's not play. If a growl is a warning of discomfort and a possible bite to come you sure as heck want to be warned first, right? Rather than be bitten out of the blue?