Steven D'Aprano writes:
I learned the hard way that if I don't put the breaking space at the beginning of the next fragment, I probably wouldn't put it at the end of the previous fragment either.
The converse applies in my case, so that actually doesn't matter to me. When I don't put it in, I don't put it in anywhere.
What does matter to me is that I rarely make spelling errors (including typos) or omit internal spaces. That means I can get away with not reading strings carefully most of the time, and I don't. But omitted space at the joins of a continued string is frequent, and frequently caught when I'm following skimming down a suite to the next syntactic construct. But spaces at end never will be.
Ie, space-at-beginning makes for more effective review for me. YMMV.