I only have experience with one setup with the 3 main channels in ceiling. It was spec audio in an upscale home. As Nick said, not good.

If you go the route you are contemplating you will at least have height channels pre-installed for later. In-walls are way better for the hidden 7.1 setup you are thinking of. (If you are surrounded by walls at least.)

With the new ATMOS/DTS:X formats about to become default codecs, and legacy decoders being the fallback, base layer surrounds are definitely priority over only height surrounds.