Traditionally (thirty years of fandom, here), slash pairings tended to be higher 'rank', more dominant personality first: Kirk/Spock, Napoleon/Illya, Blake/Avon, but Avon/Villa, Starksy/Hutch (equal partners, but Starsky having the more dynamic personality as well as being named first in the show's title), and later Spike/Zander, but Angel/Spike. In most multipairing fandoms, the 'universal' part of the pairing tended to be first: Mulder/Whoever, for instance, even though Mulder was usually written as a definite 'bottom' in Mulder/Skinner stories (I never read Mulder/Krycek, so I'm uncertain of the dnamic there). As relationships and pairings got more complex, the order seemed more flexible (see Firefly fandom).

I think because (for some unknown, at least to me, reason), Jared got first billing on the show, Sam/Dean and Jared/Jensen seemed to come about fairly organically for most, though Sam/Dean felt odd to me at first even though I just went with the flow.

But then, J2 is more common than J/J and speaks more to their equality to me, and Sam became so obviously the center of Dean's universe that Sam/Dean seems natural now. To me, the order has never been about who's more 'toppy', in a sexual sense, though.

Rambling done now.