Lots of opinions on different ways to seal the sending units so I ran it by my AP-IA. His opinion is that if the sending units come with a rubber gasket then use it. That's the way the new C-172s seal their sending units at the wing root, similar to the RV. Cessna doesn't use any sealant on these gaskets and they last for years.
The one caution he expressed was to NOT over-tighten the 8/32 screws.
-Marc