In keying data from handwritten forms, I’m struck by how badly our digits are designed: there are few pairs for which I haven’t seen an ambiguous case.
I do write somewhat defensively. I write ‘9’ with a single stroke, like ‘6’, so that it cannot be mistaken for ‘4’. I usually write ‘8’ as two circles rather than in a single stroke, so that it cannot become ‘5’ by malformation of the upstroke, nor become ‘9’ by rising too early from the lower loop. By some quirk I tend to begin ‘0’ (and ‘Oo’) by rising from the lower right, so it cannot become ‘6’.