I'd be damned if I know what fingers I use for what strings! I am totally undisciplined about it, and only get into trouble when I try to think about it!!

I play some in "3 finger" style finger picking - that is rest the ring finger on the head of the banjo and pluck with three fingers and the thumb. I use my thumb, index finger and little finger for most of the picking and my middle finger for down stroke (back of the nail) brushing. But I do a lot of downstroke with the index finger for melody or bass lines too, and the back of the thumb nail on the upstroke brush. These techniques give me a mix of 3 finger and frailing sounds.

It works for me. I do very much the same on the guitar, but the left hand is more complex on the guitar.