I pore over letters obsessively and in the past have almost exclusively offered the two leads or the juggernaut pairing in my fandoms (not necessarily to get a porn assignment, because I am happy to write gen or shipfic of any rating; it's more to try and control the themes, plots, relationship dynamics, and other stuff like that), and yet I've been foiled really often by the "requests one character, wants them written alone or with a character I didn't offer, and hates the other character I offered, who happens to be the way more difficult character to excise while still writing anything halfway decent" thing >:(

So at this point, I've given up on the idea of relying on probability and have moved on to trying for certainty. For instance, last year I only found one fandom I write in which all the people requesting it posted letters before signups closed and all the letters were things I wanted to write, so I gamed my signup and made my other three offers for fandoms there was no way I would be assigned. That was the first Yuletide where I actually really enjoyed writing my assignment, but it was kind of an extreme thing to do.