I've been spot-checking random URLs with PageSpeed and other tools to see if they find anything to complain about. Alas, they often do!
So I've been cleaning up technical slippage ... fixing validation, compressing images, minifying CSS, adding missing ALTs, tweaking for accessibility, and so on.
It's been a while since I did cleanup at that level and it's satisfying to watch the scores improve.
Will it help my SEO? I dunno, but it won't hurt!