It can be done either way, i do mine like that, i do work for one company and they want all of their trims done this way, there is nothing wrong with it.
So you are not a tiler you know absolutely nothing about what materials to use and where to start and you are confident enough too take on this job! Really.
Why are you getting the builder to tile is the first thing you should ask yourself, would you get a tiler to do the building work, you need to upload some pictures of the tiles, and what needs to be cut, can't really see there should be an issue.