Not really an answer, although useful, Trends.

I don't think they have to match, but it's often a good choice. As with any other room, you have to think about what's the star and what's the background -- often people want their floor and wall tile to be a background for their vanities or accent tiles. But you can also get a coordinated grouping of tile colors that all go together but create different feelings in different places. Just a choice.

Brag pics · More Info

Here's one where the floor and wall tile are the same, but the shower floor has a smaller mosaic version of the same tile. (I think there was a nearly subliminal accent tile to the right of the niche too, heh.)

I have been to a couple of tile stores with displays and they do not always match the shower tile to what is outside of the shower stall. However, they use tiles and colors that do match!

Allow me to clarify :)

I love the look of having the floor tiles carried into the shower flooring. This allows for a more cohesive and sleek look especially when you have a 0 point entry (curbless) shower. When you have larger format tiles, linear drains allow you to carry them through the entire bathroom.