Most of the pairings I enjoy alternate top and bottom, though some will express preference for one or the other. For example, with Clark/Bruce from DC, they go where the feeling takes them but in my headcanon Clark prefers the submissive role when they play that out. I figure it's because he has all this tremendous power and must be in control at all times, and this gives him a chance to give up that control with Bruce, who is a control freak, anyway. ;)

But Bruce can benefit from being the bottom, too, just for that reason, and Clark is the one he trusts to do that. I don't like rigid roles unless the characters are playing D/s or it's a slavefic and even then it can be ambiguous sometimes.