I am getting ready to install a Sterling shower base with walls myself ("Ensemble" made of Vikrell). The installation manual seems to say it's perfectly fine fine right on the subfloor as long as it is level to within 1/8" over each dimension and the studs are plumb to within the same 1/8". They recommend using shims if necessary. That's my plan. Also, they seem more worried about people trying to install this right onto joists than right onto subfloor, which it seems is the preferred/most common approach.
There are about 22 spots of the base, plus the front lip, that contact the floor. Also, a sheet of some special woven polyester is included, the stated purpose of which is to minimize squeaks. The manual specifically says to use woven polyester type of stuff, not roofing felt, for this.
I've thought about leveling compound, but I've never used it. You basically make what amounts to a very shallow lake of the stuff the exact size and shape of the shower base and its properties as a liquid ensure a perfectly level surface, no?
Finally, as far as movement of the base, you put in roofing nails or flathead screws in right over the lip to each of the studs. That should hold it pretty well, no? I noticed the shower base in there previously had construction adhesive long the lip that contacted the studs as well. Would this be a BAD idea?