If you want the entire sq footage of a room, you do wall to wall.

if you want to measure for tile you want to buy, you measure only the floor (or wall wall to ceiling if doing an entire wall) where you plan on putting the tile. obviously you will round up and always include 10% overage. unless you are tiling under the bathtub, it does not get included.

For bathroom tile, you usually skip the footprint under a built-in tub since tile never actually goes there — but if the tub is freestanding or removable, include it so you don't run short. Measure only the floor area you'll actually tile and add about 10% overage for cuts. I double-checked my own bathroom numbers with a quick refresher on how to calculate square footage