Sanded grout absolutely will not hurt glass. occasionally, grout might take the gold finish off of china, or change the finish on some metallic or painted beads. That is more of a chemical reaction with the grout than a sanded or unsanded issue. with any piece with large grout areas you may develop cracks. Is the surface wood or MDF? if not sealed or primed, a wood surface can suck the moisture out of the grout leaving it more prone to cracking. In addition, edges can be vulnerable, depending on the use and handling of the piece. a squared off edge like I think you are describing can crumble with a bump. Beveled edges generally work better, or an edge to the piece like a metal strip to protect it.

Have you ever tried epoxy grout? It is grout sort of infused with epoxy. It has a very pleasant finish and texture and tends to be very tough and less prone to the crumblinng or chipping of cementitious grout. I really enjoy working with it.

I might add, grout itself is very tough and strong, but imagine the edge of a brick or cement block, if dropped or struck it is vulnerable.

Sorry wackyweeder, I was out of town! Thanks for the advice. I will prime the surface and build a frame around the whole piece to protect the grout edges. I think I found the grout I want, its finish is very sandy, which I think will look pretty. -Karen