There are no classes in Dark Souls. The choice at the beginning is just for starter gear and base stats; from that point on you are what you equip. I equip armor solely for its resistances based on what area I'm in, and I never go above 25% burden. I'm not saying tattered is the best, it's just very good and very convenient.
Have you tried changing your weight around yet? There are three different roll animations in the game: fast roll, mid roll, and fat roll. The cut-off points are 25% and 50% of total burden. There's also ninja flip, but it's just an improved fast roll with more i-frames. Mid roll can be ok if you really know what you're doing, but fat roll should never be used. That's not to say you should never consider going above 50% burden, just that you should never touch the roll button when you do so.