1 do, buy sometimes it can be important to make sure you mean I and not 1 (and definitely not l) if you're writing a code of some kind.

I do, but then it's too easy to confuse with absolute value. Beats confusing it with 7 though

I'm sure some do. But not everyone does. This helps avoid the confusion.

Only when writing binary or truth tables.

I do, but dependent upon context. If there is no risk of confusion with a capital I or lower-case l (though I tend to write the latter with a slight rightward curve at the bottom), then yeah, it’s just a basic vertical line.

This is a thousand percent more than I thought I’d ever write about my penmanship. Welp.

I make my underline too wide which makes it unsuitable for anything but 1 by itself :(

But why not cross the seven instead?

My underlined 1s look like my 2s when I write with a marker. It's a problem I didn't antisipate that arose when I was trying to distinguish my 1s from my Is and ls.

But they're not phasors.