I almost never
work at a magnification of less that 5x, and even then it is very
rare for me to do much at that power. 90% of my
work on a coin happens at 10x magnification. I have gone as much as 20x for some very detailed cleaning, but it is not that common. I have used magnification up to 30x sometimes, but never for cleaning - just to see something very closely. I clean exclusively using a binocular microscope, though I have a couple of digital scopes that are basically just used as cameras.
Interesting, to me at least, is that over time I realized that increasing zoom/power does not necessary produce greater understanding of what I'm actually seeing. Granted, I'm usually working on coins that are not perfectly preserved, because if they were so preserved I would by definition not need to
work on them. That said, I often use a handheld lens sometimes between 3.5 and 5x to assist in
attribution, as at higher magnification you might get greater clarity but not necessary a greater understanding and context.
Just a few data points for consideration.
Mark