Nah, I'd say don't worry, seriously. I mean, there are periods, and sign up forms are definitely not the easiest to fill. I remember editing at least 5 times once! I'm quite flexible myself, you know? I know for sure that I love well-written Harry/Pansy, for instance, because one of my favourite authors is just a virtuose with it. But then again, Tam made Cormac/Lily Luna wonderful for me. OF ALL PAIRINGS, I mean c'mon :D So I think you might want to list what you *don't* want, and what you like besides pairings; character-driven, plotty, PWP, super angsty, fluffy, etc. Also there are many things to be taken into account for writing ; if you like epistolary smut, dysfunctional dynamics, dubcon, ambiguous endings, etc. I suggest you look at forms from other fests, forms filled in by other people (previous years, for instance!), and ideas might come to you. I think sometimes, you might want to trust people's headcanon on characters, and just indicate what you like in a fic regardless of the characters, no? I mean, I know I'd love a fic with characters I never really cared about if it's well-written (again, the Tam example, she made me super involved in fics about characters I had never read about before, Bill, Charlie, etc).
BOTTOM LINE DON'T QUIT THE BOAT AND SIGN-UP FOR SMUTTY CLAUS YOU ARE AWESOME (and if you are assigned to me I'll make you a super hot present, promise <3).