I use my index finger for N and M and my middle finger for U and I. That's the minimum amount of movement of fingers while still keeping my hand in a comfortable position.
I never learned 'official' touch typing, so while I do touch type, I have my personal style. I do use all my fingers, but mostly just my middle, index and ring fingers of both hands, pinkies and thumbs being just for Ctrl, Alt, Shift, Enter, the space bar, and one or two other keys.
When I want to type at my top speed (which I think is pretty good for an untrained person ? best was 95 wpm without mistakes on speedtest.10-fast-fingers.com/, although my average is more around 80-85) then I only really use my index and middle fingers, except for shift, space, etc. I don't know why that speeds me up, but okay.