It varies hugely. Of the 3 local options, Aldi and Tescos (both edge-of-town) have the racks hidden away, we don't want to bother any customers, do we? So of course I park somewhere visible.Not sure what your supermarket is like, but of the ones I frequent, bike racks are very visible in a busy area, I've no qualms leaving a nice bike there.
Sains has them out front, because they're part of a shopping centre that was designed around a central seating area (kinda). I play by their rules there