[pitspotter]

After sight-reading through the piece a few times you can generally start writing down the fingering on the score. Keep a pencil handy.

It's a bad idea to finger on the first play or the first day because there's a risk of premature optimisation. It doesn't take much analysis apart from one or two specific problems. The fingerings I end up with usually vary somewhat from the editor's.

But the OP is correct, once you've got a fingering, you have to stick to it, even if it's suboptimal in places. It's your fingering, and if you change something after 200 repetitions there's a danger of slipping back into the old pattern and stumbling.