Just a bit of useless trivia: I grabbed the 635 original Amazon books I'd bought (that I still had copies of) and brought them into Calibre with KindleUnpack.
- Of those 635 books, 215 had embedded fonts.
- Of those 215 embedded fonts books, 30 had embedded fonts in the actual text (i.e., not general body fonts and not in the advertising pages).
- Of those 30 actual embedded font books, 1 (one) used fonts in a meaningful way (just barely). And, I don't think that use is worth my embedding fonts in my edited version.
Now, those Amazon books are my older books (from before Amazon locked everything down and forced me out of their ecosystem). I still have to do the same for my newer epubs from other retailers. But, that looks like only 45 books. So, my blindly stripping embedded fonts out of books doesn't seem to have done me much damage.
I'll be checking embedded font use from now on as I re-format all my new books. But, I'll guess that I'll be stripping any such fonts out anyway. Thankfully, doing that check takes hardly any effort when I'll already be in the book replacing all the CSS.