To me, it doesn't matter what the writers mean, it only matters what shows up on DVD.
Ted is clearly and repeatedly shown in both positions -- he fucks Troy AND gets fucked by Troy, he fucks Emmett, he is fucked by many other guys he has sex with -- I don't think Ted identifies himself as a "top."
Emmett clearly has a "bottom" identity, but even so we see him challenge that identity more than once.
We don't know about Drew. We can assume, and I would also assume he'd not let himself get fucked in the ass, but then it's just circular logic: Drew wouldn't get fucked because of his character, and his character is of a guy who won't get fucked.
Ben clearly fucks and gets fucked, Michael says he's "not a total bottom," Emmett and Justin do both, Blake we have no idea. I still do not like creating a characterization system that uses a sexual position preference to tell us something about a character, when what we're really after is the characterization itself.
Ted and Justin might like to fuck and get fucked because they both feel good, and there is no identity issue tied up with it at all. Ditto Blake or Ben or, to a lesser degree, Michael. Those things might tell us something about the characters, but it could be something as simple as, "Likes both cock and prostate stimulated."
Brian does seem to self-identify as a "top," but it's presumably due to his own psychological issues. And it's the psychological issues that comprise his characterization, not a sexual position. Although I agree that for Brian, this is a genuine part of his characterization. But it's not universal to other characters or to the concept of characterization in general. When you make it be so, IMO you're relying on sterotypes instead of actual characterizations.