I have always found the niches to leak. we use radius corner granite shelves instead. If you do use a niche try to put it out of the way of the shower. I would also suggest you use this pre fab niche (see enclosed). it can be tiled over and it seals up pretty good.

You should be able to remove them so yes leave enough room for that

https://www.tileredi.com/niches-and-decorative-shelves/redi-niche.html

Thanks for all the comments - I was thinking that they should not be removable (to avoid leaks), but I don't want to be cleaning all those corners, which is why I was thinking that there should be a space 1/4" between the back edge of each glass shelf and the back wall of the niche. I would have 3 shelves in one long niche. Thoughts ?

I think Ironwood had some great ideas with safety glass and not being removable. look at those niches I posted. they are supposed to help with leaks. send pics. I would love to see it done

homecrew, Better with the glass set! If you have hard water...get a water softener!