I just bought and used a bottle of Liquid Plumber but I was just worried about whether the stuff might be able to corrode the rubbery seal under and around the top of the bathtub drain. I live on a second floor apartment and the walls of my shower are already corroded enough to leak below, but that was patched. I meanwhile have a lingering phobia of leaks and was just wondering if I should be concerned about this seal. I can visibly see the seal. My apartment--a condo which I "own"--is about 18 years old, as is this bathtub.
I think the answer is going to be "it depends on what that rubbery seal is actually made of" -- here's the quote from the Liquid Plumbr FAQ --
Is the part with the rubber seal removable? If so, remove it before you use the liquid plumbr.