Always been B. I mean, sure, it happens that one Pokémon outlevels the others for 2-5 levels, but I don't mind. Then, I just train the others so they won't be left behind. A level or two won't hurt anyone, though.
Well, it seems that Type B (the well-rounded one) will be the most common, because wouldn't most want to have fun with as much Pokemon as possible? Yes, I am that Type. Since Pokemon Diamond, I started planning my teams before I play, so there is some effort put there, so I guess you can say Type E is touched, except, I never go and train to insane levels, because that takes too long. I remembered that my Diamond team was sandstorm-oriented with Weather Ball Roserade and of course Hippowdon.
B, to the point of deliberately training pokemon I'm not gonna use to keep levels low enough that the next one I catch won't need massive amounts of training. Everything evolves for the first time at level 55 - I don't care if it's a Starly or a Beldum, they are't allowed to evolve until then.
Around the age of 8, however, my team in Crystal consisted of a Level 100 Typholsion, a Level 100 Feraligatr from Gold, a ndLevel 100 Charizard from Blue, so type A.
A very strict type B. I train each Pokemon one at a time; when they advance in a level, I move on to the next Pokemon in the list. So my Pokemon are always within one level of each other. My current team in Pokemon Emerald are all Level 56.