Usually the metal corner bead should provide the perfectly straight line and the tile grout is supposed to end at that bead. If not, I will either paint the grout right up to the tile or make a straight line by hand following the grout line. You can also use tape.

carrie, regarding the blue tape, it comes in various degrees of stickiness. So if you're concerned about pulling any grout off, consider looking for the blue labeled for "window glass" use. Just a tip I learned while watching window renovations-thought it might help here. GL.

Very carefully with a QUALITY brush and a steady hand.

Thanks for the good advice everyone. I was thinking about asking the tiler to end the tile just shy of the corner to maintain the sharp corner. I know how to change colors on a sharp outside corner by tapping a lightly loaded paint brush sideways along the edge. Would that work, or would it just look bad?

I have two corners like that, and I had the tile installer make a sharp edge on the grout, bringing it even with the drywall on the other side, and just painted the edge of the tile and the grout the same color as the wall.

It's almost invisible