Nooooo
That would definitely not work in the scenario I gave.
You would probably find that in your scenario, say if the wall was 6ft in height at either end, and the middle. And the room/floor runs down 6 inches though say. You need to add 6 inches on top of that measurement you took in the middle. (As there would be a 3 inch difference perhaps from a level line, both above the line and below it.)
Though in the same room if it didn't run off from one end to the other 6 inches only, but it ran off 6 inches from a datum line in the middle for the top of the wall/room, and 6 inches from the heigh and low point of the floor too, your room would be 12 inches greater than taking a measurement all the way around the room that is still reading 6ft.
If that makes sense (it is late!)
If you just do what you said on your jobs, and you had that (admittingly bad - but bad to give a good example) room, then I'd be surprised if you said you got the tile qty right on any highish wall job on an oldish building with large tiles in a bathroom say, and one that's of some fairly large size. Those are the sorts of walls that you need to think about a lot during measuring.