Also a newcomer to mtb, I'd like to hear answers on this, too. When I purchased my mtb, the shop did a cleat fitting with the installation of clipless pedals. The resulting position seems to duplicate my road fit in terms of saddle height, but I often wonder about this. It feels efficient, but I am inclined to try lowering the saddle by an inch or so while learning bunnyhops and the like. I hear some folks like to lower the saddle for descents, simply to make getting behind the bike easier, but I don't foresee this being as big an issue for me.

I have yet to log much actual trail time, having purchased the bike in the dead of winter, but I kind of came to the opinion that I should go clipless from the start on the mtb (already clipless on the road) to make it second nature to me. I have SPD pedals set to the lowest tension, and they have enough of a platform that I can do a half-in/half-out stance if I get too nervous about a section of terrain.