I was thinking this through and wandered whether high ISO really mattered from a pure IQ perspective when shooting in RAW? To the best of my knowledge, ISO is an digital exposure boost applied by the camera using a proprietary algorithm. Whats to say 1600 on a 5DMKII pushed two stops in LR isn't any worse than 5DMKIII at 6400?
I started a thread about this a while back and gave examples...the short answer is no...but...
Apparently there is a detail threshold and staying within this is the key.
I shot many shots and a 5 stop pullback from an ISO200 shot looked pretty much identical to the same scene shot at 6400.
It was a very interesting test but the conclusion was: if you shot in manual within your cameras threshold for detail, you can pull back in PP. The only thing is, obviously it won't be spot on in camera.
EDIT: sorry, just read this back and it didn't make a lot of sense. I'll try again...
A few months back we were discussing ISO in another thread and somebody posted a link to a camera rest on a D7000 which showed something ABOUT DSLRs that if you shoot within the camera's detail threshold (can't remember the exact terminology) that the body simply applies, like you said, a digital boost to the signal. This was proved that shooting below a certain level (for the D7000 it's ISO800) will give you the same results as shooting at the correct ISO to begin with.
I was intrigued at this new knowledge to me and took a few shots indoors at ISO's below what they should have been and also another shot at the correct ISO.
The results were bar a little contrast reduction in the pulled back exposure, they looked the same.
There shots went all the way up to 25600.
Conclusion for me was:
In theory, you could do a wedding by sticking your camera at ISO400 and just leave it there in manual all day. The results when pulled back in PP will be the same.
Obviously, that's not an efficient way of doing it but what it does prove is not to be afraid of pulling back and IQ loss when you didn't nail the ev. I tried shooting and ISO3200 to start with and then boosting and the results were considerably worse.
So my simple answer is no it doesn't matter (unless your ISO is higher to begin with) but for obvious reasons, it's best to up the ISO rather than shoot low ISO and boost in PP all the time.