Shower tile inside corner vertical joint
Inside corners should always be caulked, not grouted. Walls move, and when grout moves it cracks. If you're using epoxy grout on the walls you can actually use it in the corners too but keep in mind it's more difficult to work with especially in corners. If you're using epoxy grout on the walls and going to caulk the corners, epoxy grout the horizontal lines first and then caulk the corner after the grout sets.