When you evolve a Pokemon, the point on the CP bar above their heads is constant. So if you evolve a 1st stage pokemon and it's at 50% of its CP bar (dead center at the top), that's where the CP bar will be after evolving. You don't get any power advantages for waiting until after it's evolved to power it up. Whether you power it up before evolving, or power it up after, the results will be the same.
However, it is more cost effective to keep a revolving copy of each pokemon. For example... If you have a CP30 Rattata, instead of powering it up, keep it in reserve until you have enough to evolve it. The chances are good that you'll find a better quality Rattata before you save up enough candies to evolve. When you find another one, Transfer the weaker of the two, and repeat that process until you have enough to evolve. That way you will get the most bang for the buck, and won't waste stardust that you may need later (and quite often you spend less candies that way too).