My strategy is to wait.
Consider the scenario in which I catch a low CP Pinsir (or any decently rare Pokemon that can't be evolved), something like 200-300 CP. If I am, say, level 20, the maximum CP for Pinsir is 1324, as stated here (you can find max CP per level for all Pokemon at this source). If I want to power-up my Pinsir all the way to max, each power-up would raise its CP by the same amount as my current level. I would likely level up in the process of powering-up Pinsir, so let's say that each power-up would raise Pinsir's CP by 21 points. This means that to raise my Pinsir from 250 CP to 1350 (accounting for a bit more in the max CP due to my leveling up to level 21), it would take:
#powerups = (1350 - 250) / 21
It would take me 52 times to power-up my Pinsir.
Stardust aside, you can see from here that it would take 2 candies to power-up my Pinsir if I'm level 21. This means it would take 104 candies to complete all my power-ups. If I can get 4 candies per Pinsir I catch (3 for catch + 1 for transfer, already assumed to have 1 starting Pinsir), I would have to catch 26 more Pinsirs.
If I have to catch more than a few more Pinsirs (and this goes for most rarer Pokemon that don't spawn as commonly), it is far more likely that in the many more Pinsirs I catch that one of them will have a much higher CP. Thus, it is better to wait.
In addition, the stardust demands for doing this are quite extreme, and also lean in the favor of waiting for another Pinsir. While it might seem frustrating to catch a very rare Pokemon to only have it be very low CP, you will have to catch more of that rare Pokemon to power it up, so it is just better to wait in most cases.
A brief side-note, I usually choose a Pokemon to commit to and power-up when it's "power bar" is pretty close to max, I am at least Level 20, and it is a Pokemon that is proven to be top tier.
EDIT: As noted by @EBongo, a Pokemon's max CP increases as your trainer level increases, so if you catch a Pokemon and it's terrible, it will only get worse (another reason to wait for a better Pokemon).