I'm trying to finish up our shower. We ended up using a Tile Redi (which was an ordeal, but that's another story...) which means that the pan, itself, is completely waterproof, and that the tiles are epoxied to it.
For the shower floor, we used hexagonal tiles and grouted them.
For the curb, we decided to go with large format slate tiles. As such, we have only a few gaps to fill (2 vertical, one horizontal) along the entire 5' curb. As the gaps aren't large, I can't use sanded grout and it occured to me that this might actually be a lot easier to just caulk. They are (nearly) black tiles and I'd use black caulk.
Is there any reason NOT to do this? As the curb won't be getting much direct water, I'm thinking caulk will last just fine on it.