IMO the better the tiles the more you want.
If you pick super cheapo tiles or ones that are going to date terribly, keep them to as few as possible. The more you spend on the tiles, and/or the better they are, the more of them you want to show.
Mine are all part tiled, downstairs loo, 2x ensuite shower rooms, master bathroom. I will only be tiling the main bathroom fully when I refurb them. The ensuites and loo are just do what you need to and go, although master ensuite i think could possibly get more tiles than it has now. Where as the bathroom I feel will benefit from a whole tile layout, as when we go in there for a bath we want it to be more luxurious. The same logic as getting an expensive bath tub to lay and relax in, as opposed to an expensive shower where i am trying to keep shampoo out of my eyes and will be in and out in around 5 minutes.