I was just about to install some towel holders on tile in the bathroom of our recently purchased house, and noticed that the tile is strangely not installed parallel to the floor and ceiling. The tile lines slope 3/16" over the two feet of a towel holder, and almost 3/4" over the whole bathroom. The slant of the tile is not really a problem in itself -- you would not notice with a casual glance.
But I wondered what was the best choice for aligning the towel holders -- level with the floor and ceiling, or aligned with the tile, at an 0.8% slope, or something in between. It's not a technical question; more a matter of aesthetics or principle. What would a pro do?