The Tile Council of America (TCA) is the basis of the codes on shower installation for most of the country. They say do not grout the changes of plane in a shower (i.e., the corners and the wall/floor interface). Instead, use caulk. This is because few showers will be expanding and contracting the same between those corners, and the grout will likely crack. If your shower was built very carefully, you might get by with that and not have it crack, but that is actaully rare. If you've already grouted them, leave it alone and see. If it does crack, then you'll need to remove the grout, and replace with caulk. Caulk over grout never works for long, and usually looks lousy. You can buy caulk color matched sanded or unsanded grout for nearly any color grout; you may have to special order it, or get it on-line, but it is out there.