My understanding is that you use caulk where there are 2 different surfaces coming together & those surfaces could move. For instance, where the wall meets the floor of a shower or the edge of a tub. Caulk can flex a little bit so if the house settles or you bump a heavy object (or yourself) into the other side of the wall, that little movement would crack a rigid joint (like grout) but won't crack caulk.

Grout is more durable & if the wall moves it will move as a single unit, so grout is great there. But where you have 2 planes or 2 independent surfaces coming together, you want to use caulk.

Thanks for the response. My tile guy grouted the corners as well, so I guess I will see if I get cracks over time and caulk if they appear.