I've been reading up on how you create a shower niche and if I understand this correctly, you create an oversize waterproof niche opening so you have the ability to adjust the tile as necessary to match the surrounding tile. I'm going to be tiling a new shower next year and also want my tiled niche to match up exactly with the surrounding tile and like this approach. What I wanted to know is the best way to fill in the approximately 1" (plus or minus) gap behind the tile. Instead of filling this gap with thin set, would it be better to grout in a shim between the niche and the tile? If water behind the tile is a concern, I could make this shim out of a non-porous material.