I sanitize and charge my cornies with CO2 before putting them up for storage, leaving a little sanitizer in the keg (I vent a little through the out post to suck up sanitizer through out tube). Then when it is ready for use, I simply vent it and pop the top and rack into the remaining CO2 as a layer that rides above the beer as it is filling. I rarely re-sanitize, but have done so when the keg sat for an unduly long time or if there is any question about the seal holding during storage.
I always resanitize my fermenters before using, since none of them have been held under CO2 during storage.
So far, so good.