As soon as you lift a tile to cut it then its taking you longer to fix that tile so you charge for it.
I'm the same as you Andy,
Measure room at 20m2 + 10% wastage = 22m2, that's what gets invoiced...
Seemples.
Like I said previously, it's easy to tile flat walls, It's when you come to the cuts that's where the skills are, and they need paying for.
We've seen all too many times on here what bad cuts look like and they make or break a job. If you choose not to charge in that manner then you are missing an opportunity that's staring you in the face.