I have a similar question along these lines. Does Evolving really matter at all? will a 10CP rattatta still become as powerful as a 150cp rattata when they become evolved into Raticate? OR will the Higher CP give me a higher starting point and thus a higher CP at peak?
Each Pokemon (and their associated evolution stages) has a set Maximum CP. When you evolve a Pokemon, if you take note of the position on the curved bar above their head, you'll see that after evolving, that bar doesn't change. ie.. if the CP bar is dead center at 50%, after evolving the CP bar will still be at 50%. So basically, if you have a 1st gen Pokemon that you powered up until it's maxed, you won't have to power it up anymore after evolving to higher stages (you won't even get the option when it's maxxed). Additionally, the candy/dust cost for power ups doesn't change from 1st, 2nd, or 3rd stages of evolution. So if you wait to power up until after evolving, the power up costs will remain the same. You won't be penalized for powering up the 2nd or 3rd stages just because the CP went up. It's soley based on where you are on the CP bar.
Each breed has a set max, so if you're concerned about CP it doesn't really matter at what point you evolve it because the max will be the same regardless. The only consideration you may wish to take into account, is candy/dust cost efficiency. For example, if you have a CP 10 Pidgey and a CP200 Pidgey each of which costs 12 candy to evolve to it's 2nd stage, evolve the CP200 Pidgey so that you can bypass having to spend a ton of dust and candy powering up. The end result in power will be the same when they reach max CP.
Another example, I only just today got my 5th Pikachu (hatched it from an egg) that has the CP bar at almost full (probably only needs 3 or 4 powerups to hit max). Which was a great relief, because for me they're so rare to find that I thought for sure it'd take
ages to save up enough just to evolve it, let alone power him up. I probably shaved a couple hundred hours of farming Pikachus, and a boatload of dust, with that acquisition.