It's not necessary to mount them, they'll encrust out either way. My fastest growing ones are the ones I glued to rock that they never attached to. They kinda plate a half inch or so over the rock and grow slighty faster than ones fully encrusted on the rock. I grew them for years on the sandbed and they did fine that way too. Frag discs help if you have any critters on the sand that like to bulldoze things over.
IME they don't do well on sand, because even the slightest buildup /shifting of sand on the flesh, and they will receed. They aren't adapted to deal with sand piling up against them, like other corals are. Obviously if your sand doesn't move, then I guess it's OK.