I think going on at length about what you don't like about a particular pairing is probably unnecessary, but a brief, non-judgemental note about why (a simple "I like the canon couple, and would rather it not be broken up," or "incest isn't really my thing," or whatever is applicable) probably isn't a bad idea - and knowing why you don't like a particular pairing, rather than just knowing that you don't, might clue your writer in to some of your general preferences regarding the fandom, too.