Everyone will have to make their own decision on this. Personally, I would pay the extra $20 they may cost later to avoid the opportunity for anyone to use a poor intrepretation of the law against you.
However, for someone who wants to prep for post-ban times, the risk is negligble. Look at the SEBR forum about Armalite's Post-Post ban rifle program -- they'll be sending out flash hiders and information kits to registered owners in advance of the sunset if expiration is imminent, so you can pop it on 9/14 if you're so inclined. If Armalite doesn't see liability in this, I don't see it either. They'll even sell telestocks 'in advance' when the time is right!
if the gov is out to get you, they will. If they don't find something wrong with your guns, they'll find something. Did you mailorder anything last year? I'm sure you claimed your local sales tax on your year end and paid the state, right? You don't want to evade taxes. You don't own more poker chips than you need, do you? Or 50 decks of cards? Conspiracy to operate an illegal gambling establishment is not good. (God forbid you have even a legal slot machine in your game room).
I'm sure if one chose to stock preban parts in anticipation of a ban sunset and didn't give the gov a reason to check in on them, it'd likely be a negligible risk. However, some interpretations of the law indicate that the ownership of 1 post ban AR (complete) and one pre ban AR (complete) gives the gov the opportunity to claim you are possession the pieces to assemble an illegal configuration. I myself am squeaky clean. I am 100% postban and satisfied for now. If you want to eliminate all risk of a violation, this is the only 100% risk free position to take.
Like driving over the speed limit, you must gauge your sense of the level of risk and the benefits of ownership and make your own decision.
-Dan
PS -- Even in MI, I was considering buying a FAB10 so I could get a preban styled rifle for fun. I decided not to because of the cost (A $300 receiver?), but I know owning both would create that miniscule risk I mentioned. Like you, if the ban stays, I'll have to make a decision. Likewise, when I finally purchase a M16 under my C&R (the legal route in MI), I'll have to decide wether or not to shake loose all my semi-auto rifles for the same reason...I'd technically have all the parts to assemble an illegal config. I know a guy with 5 uppers for his M16. 7.5"ers, 9mm, 22lr, 10.5"er, etc. He only has one receiver. He understands the risk and made his choice, and uses them only in legal configs. Right or wrong, it's his decision.