I think it all depends on the quality of work, and the expectations that you have. For example, if you hire a crew to renovate your bathroom, and they do a decent/good job then i don't see why they should get a tip. However, if they go above and beyond, and give you good suggestions, maybe about relocating something, or about putting the tiles in a diamond pattern instead of plain square pattern, and the end result comes out much better that you expected/would of been with your original layout then yea they deserve some kind of tip. Same goes if they do work for free that is not in the scope of work. Such as re plumbing a fixture, or relocating a door, extra accents and borders when tiling.