Thank you guys, I appreciated your answers!
I think I'm gonna evolve my "story" pokemons and I'll keep not-evolved those who are gonna have a place in my "dream team" for bosses fights and/or tournament, the ones I'll train with proper nature and evs in short.
I've always seen pokemon adventures as a funny path during wich everyone should enjoy his team entirely, watching it growing (and evolving, lol) fight after fight. For the competitive part of the game, I completely agree with those who state that is better to not evolve them, just to make the boring leveling part a little bit faster.