The underlying concept is to keep laminations as "matched" sets of material having the same expansion coefficient - like properly-made plywood, a core encased in matching sets of veneers.
MDF doesn't have a conventional expansion coefficient, but is subject to moisture absorption, which could induce some warping. Tempered hardboard would be subject to the same sort of effect. If you don't want to apply Formica to the bottom surface, I'd suggest sealing it with a couple of coats of shellac, and then wax it if you want. Shellac is an excellent moisture barrier.