Like Stretch mentioned...it doesn't matter. Buy the new MBP with the retina display now...or later. Apple does say that you get a free upgrade to Mountain Lion when it is released...and even if you didn't...it's only going to cost $19.99!
As far as Mac's running best on their "native OS"...or original shipping OS. Well there may be some truth to this...in the sense that as newer OS versions are released (many times they get more complex)...and thus need more cpu & gpu "power" to run well.
But guess what?? If you purchase the new MBP with the retina display...and eventually run OS 10.8 (Mountain Lion) on it...it's not like when OS 10.9, 10.10, 10.11...or OS 11.0 is released that you won't run it on this computer.
So it's really not much of a discussion point with the new MacBook Pro and OS 10.8. They will run well together...and so will OS 10.9, 10.10, etc.
- Nick