if your book requires non-ascii characters -- i.e., characters above the lower-127 -- use unicode. if your book works without it, as most english text does, don't think about it, just consider yourself lucky and be happy.
if your book is in english, and you are using some words that we've borrowed from other languages that have accents -- such as "cafe" or "vis a vis", and so on -- you can just leave the accents out, as we know what those words are and what they mean, even when they are displayed without their accents.
use regular quotes, not the fancy high-bit curly-quotes. (if a user wants curly-quotes, there's an option for 'em.)
use a double-dash (like this: "--") not the high-bit em-dash. (that's another option the end-user can choose if they want.)
in general, use only low-bit characters, not high-bit ones.