In my opinion, a sortie starts when your wheels leave the ground and ends when you're dead or when your wheels touch the ground. If you hit the rearm pad, once you see the message that you're rearming, you should also "land" any kills as if you towered out. Once you taxi off the pad and your wheels leave the ground, you have started a new sortie.
Same could be said about resupplying a GV when he runs out of ammo. Once you take supplies, the "sortie" ends and you "land" any kills up to that point.
I think one of the main reasons people rearm multiple times is to rack up a bunch of kills to see their name in lights and get all the "WTG on landing 20 kills", rather than helping their rank and score (since rank and score is already too easy to artificially manipulate).