It's not your @font-face rules, it's the font-family attribute in subsequent rules in your OPF.
For instance, if you had
This would get nicely overridden by the user style sheet to be what the user actually selected.
If however you had
This isn't currently being overridden by the user style sheet. The result at worse is a ransom note effect with the fonts being a mix of the author (your) selected fonts, and the user selected fonts.
Putting the font-family attribute in the least specific selector (such as p, div, or body) that you can get away will potentially avoid this.