What I like about this fandom, is that in the many fics I've read, even when the characters do not switch, it never ever determine their personalities, something that cannot be said with yaoi - and I mean yaoi, for instance I've been writing in manga fandoms for a long time and it was never about a seme/uke relationship, the approach was more Werstern-like - were the uke is really a whiny girl with a penis, and the seme a scary rapist - yes I like manga and slash, no I don't like traditional yaoi - , so I thought this was really nice for once. What people like to do in their bedroom usually doesn't have an impact on how they behave outside of it, with maybe the exception of bdsm, it's just a matter of preference. So yeah, I don't mind the characters switching, but if they don't it doesn't bother me much, as long as they enjoy themselves.
However, it is true that I may have a preference for Steve/Tony, and the reason behind it is... kinda weird. I've noticed that in every pairing I've been into, whenever one guy has fair hair and the other dark hair, I always love when fair-haired tops and dark-haired bottoms. And I have no idea WHY, because how odd is that?, but it's probably a subconscious thing. So, yeah, as ridiculous as it sounds, I like it better when Tony bottoms and Steve tops, because of their respective hair colour... (Hence why I also prefer Watson/Holmes, John/Sherlock, Naruto/Sasuke, Remus/Sirius, and the list goes on...)