It's pretty even in D/C fandom, tbh. People will say there's more of one than the other, but really it's got a good mix of both. Plus it goes in waves - there'll be more bottom!Dean for half a season, then more bottom!Cas in the other half. I think it largely depends on which of them was a jerkass to the other roughly 3-6 months back in canon. Fandom will make the jerkass top more often, it just takes it a while to catch up to canon.