I've been doing this for a while now. It's nice to get the keg all cleaned, sanitized, and sealed up a few days in advance so you know its holding pressure and you don't need to adjust the seal. For those few days I leave some star san in the keg, and right before filling I evacuate it through my racking cane and everything to give it that last minute sanitization before I plunge the cane (with cap already attached) onto the carboy