Faucet towers typically come with a neoprene "gasket" that fits inside the flange and has punch-out perforations for the flange screws to pass through. My 6-tap t-tower with a 3" column came with one and I dutifully used it.
But, frankly I'm not sure what the point is. When you actually stop and look at the whole thing, that neoprene thingie is not going to stop liquid from getting under the flange - indeed, it'll likely soak up and hold onto it. But trying to apply any kind of sealant around or under the flange would likely make an unsightly mess.
At most the gasket will minimize air leakage around the tower flange, which is an ok thing, especially atop a keezer where moisture can be a beyotch to manage. But I don't see much more benefit than that...
Cheers!