Short of opening up each book, I'm sure they'll find out like I do... when I open the book to read it on my Glo HD.
As for how many bother to then open up the book and fix the spacing or fonts or whatever, only those readers that it really annoys. Like me
. Those who don't bother to fix it are the ones it just really doesn't bother, because it's not difficult at all to fix and takes just a few minutes even if you do it manually in Sigil.
Still it's a common problem with most readers that I've ever tried, not always being able to override a particular item within the book's stylesheet.