I will always defer to John Bridge forum on tiling issues. here is an excerpt from his forum...
The industry handbook, the TCNA publication, calls for all changes of plane to have a movement accommodation joint, and in a shower, that is often caulk, but could be an engineered joint (grout is not).
Now, some installers grout their changes of plane. Sometimes, they get lucky, sometimes they don’t. It may not ‘fail’ immediately, and can take many years, or you may never see it. Depends on a lot of things like temperature swings, humidity changes, direct sunlight, heat ducts, and probably some other things mostly related to heat, but moisture can cause things to swell and shrink, often between seasons.
Since grout is usually the weaker substance, the grout tends to crack first, but in a worst case scenario, it could also debone or crack a tile as well. Note, getting the required thinset coverage on a larger format tile is a lot tougher than with a smaller one, and that may make deboning more likely through movement.
Cracked grout should not cause leaks if the rest of the shower is built properly.