Another rental property question. (I always say that because the rental
rehab we acquired had some *sloppy* el-cheapo construction. Wherever I
can I try to improve as I fix.)

The city noted on its inspection a loose toilet (base). There isn't any
obvious leak, and the nuts seem as tight as they can go, but it rocks
front to back as much as 1/4".

Apparently there's a way to level toilets using "metal shims" -- is
there a special kind I should get? Does it matter that they're metal
since it's on top of vinyl flooring and plywood beneath that? If I shim
it should I seal using something like plumber's putty (or would that
conceal and concentrate any rot?). If it isn't leaking, do I need to
take the thing off and replace the wax seal, or should I just do that
anyway?