I agree with madcratebuilder.
I think that "constructive possession" is a boogeyman. People love to trot it out and scare each other. The people that get charged under that theory are people who are on the ATF's radar for a good reason, and it's usually in relation to illegal full-auto stuff.
I would never suggest doing something illegal, but I don't think you're at great risk by having a stock available to put on a rifle as long as you don't do it. First, you're not being investigated by the ATF already, so how would they ever know? Second, you haven't actually put the stock on the rifle, so there's no actual violation of law. Third, if you've applied for a tax stamp already, I think that shows an intent to follow the law, not violate it. Fourth, if you have another firearm it can go on, then there's a reasonable explanation for why you have it.
With all that said, I usually err on the side of telling people not to have a stock around if they have an AR pistol as long as they don't have other ARs, because it's the difference between a miniscule risk of punishment versus no risk.
The advice you usually hear is this: make sure that your quantity of parts matches up in such a way that if everything was installed on every possible weapon, you woudn't have any illegal configurations. That applies to stocks, short-barrel uppers, etc.
So if you own a full-size AR with no extra parts, and then an AR pistol, there's no problem. But if you own a full-size AR, an extra stock, and then an AR pistol, assembling all the parts together would get you a full-size AR and an SBR AR with no extra parts. Potential problem.
Same with short barrel uppers. If you own a full-size AR, an NFA-registered SBR AR with an upper on it and one extra short-barrel upper, then you're probably okay, since you can easily say that the extra upper is only meant to trade out on the NFA-registered lower. But if you own a full-size AR, an NFA-registered SBR AR with an upper on it, and then an extra short-barrel upper and an extra un NFA-registered lower with a complete fire control group and stock, then it looks like you really have an unassembled illegal SBR.
Again, this is all theoretical. Unless the ATF has some other reason to kick down your door, the chances of being charged over this kind of stuff is practically non-existent. The "constructive possession" theory as it relates to NFA firearms appears in cases where they busted someone for other, bigger charges as well.
That's my 2 cents.
Aaron