The challenge is if you run into an overzealous ATF dude and he decides that having the short barrelled upper in your safe next to any non-registered lowers constitutes "intent to assemble."
I have yet to run into any ATF dudes (to my knowledge)... and I don't show people the inside of my safe.
Shit. I had an 11.5 upper stashed in my locker for a long time before I got around to registering one of my lowers.