Thanks everyone. My tanks are newer, not even a couple years old. The Paterson tank lid has no threads -- it's the "Tupperware style" lid mentioned above. It takes some time to get it on properly -- time that should be spent agitating -- and I think a thin layer of something (like Vaseline) would help it pop quickly into place. Maybe. Even so, brand new ones are prone to slight leakage. I have two of them, and it's the same with both.
Also, one of my Unicolor drums has a slow leak. I've read that a little bit of Vaseline around the gasket will slow or stop the leak.
I appreciate the practical considerations, but mostly I want to make sure the goop won't retain chemicals over time and processing cycles -- or migrate to film or paper if used very sparingly.