I have this question also. You are right that there has to be a gentle slope toward the drain, but then the tub is not on a flat plane and is also sloped... we have this similar issue to deal with.

in our wetroom design, the shower area is a distance from the tub itself. i don't think i would be wiping it down every time i shower. do you guys dry out your tub every time you take a bath?

and if the wetroom is properly drained and ventilated, why do you think this would collect mold etc. more than other situations?