Good practice. If you don't word-wrap plain-text emails, each paragraph will show up as one long line; the program reading your mail on the other end can't wrap it automatically, because it might represent code, ASCII art, terminal transcripts, or something else that shouldn't get wrapped.

So, wrap your paragraphs in plain-text email at some sensible column. Common convention suggests 72, because that allows for a few rounds of quoting before it passes 80.

This might not actually be that off-topic, because I think the underlying reason that git commit messages are stored line-wrapped is for ease of sending them unmolested through email.

I think so, yes. Certainly, something like this is very common in Mutt configuration files:

    set editor="vim -c 'set tw=75 ft=mail noautoindent'"

The 'set tw=75' bit, sets the text width to 75 columns.