Good question and I'm in the exact same quandary. There is lots of chatter on this subject on the various forums. The otherwise very clear Black and Decker plumbing book shows pouring a layer of thinset before placing the receptor but warns against putting mortar where the receptor feet contact the floor. The pan illustrated is exactly the same as mine (Kohler/Sterling) and has numerous feet on about 6 in centers. How could you contrive to put the mortar everywhere but under the feet? (The directions do say to put a 1 in layer of mortar down, as an option.)
I will need to shim to level the receptor/pan and I do see that when I do this the pan will rest on the shims with some of the feet a bit shy of the floor. So maybe mortar/thinset makes sense, but only if it spreads and sets under the raised feet. I really don't understand this and I hope someone responds to our posting soon!