If you are going to build the Niche - you should frame out of 2x4s and whatever it takes to make the depth you need. It should sit flush with your backer and the seams should be taped and thinsetted. Use the tape made for backer, as you should use in the whole shower. Go the extra mile with Redguard in this area, try to seal as much as you can! When you tile it, pitch the bottom very slightly toward the shower for water to roll off. Do not put the niche too low or in direct stream of the water - so it doesn't fill up constantly.