I would do all walls, because I think it would look weird not to.

Put up the crown molding first on all four walls and then tile up to the crown molding on the one wall.

Thanks for the comments. I think the suggestion to install the crown first and then tile up to it is spot on

IMO large crown in a tiny PR is a bit overkill and unless you have that crown in the same size in the rest of the house I would not do it in the PR and I would need to see the tile since it is going to be the focal point maybe that is all that is needed. so a pic of the space one of the tile and one of the crown you want to use.

There was crown molding in my living room and foyer when we bought our house two years ago. My powder room is down a short hallway off the foyer and the living room opens to the foyer, so the rooms are close to each other. My husband put crown molding in the powder room last year when we repainted. He used a reduced profile in the powder room so it's not as tall as what is in the living room and foyer. It looks great - we have gotten compliments on it. It classes up the room. :-) So as Patricia says, if you do crown molding it should be in keeping with the rest of your house.