Is it ok to leave gray and black tanks open on a full hookup site?

People are very split on if it’s OK to leave gray tanks open or not. The folks who do it generally haven’t had problems. The other side says you’re opening yourself up to sewer flies, a mess of the campground’s sewer reverses or backs up, and that your possibly venting the campground’s sewer system.

I leave mine open but a friend who has experienced sewer flies says that once is enough for him with that and he’ll never leave his gray tank valves open again.

And always leave black closed.