an inward rolled lip? As in, toward the interior of the pot? Strange.
If you were just boiling water, I doubt a lip on the interior would grow much, due to a lack of medium. It's been a while since I took microbiology, but if any food got stuck on the interior you could definitely grow some undesirables. The food could act as a source of nutrition and as a protective barrier, and some types of gram-negative baddies can withstand boiling temps for up to ten minutes IIRC. When is the last time you boiled water for 10 minutes with an alcohol stove?
Anyway, thread drift.