I guess I should have expanded on that
I hold my ARs by the front of the magwell. Aside from the non-effect on zero, it's also universal in that it works for every rifle you pick up. I've shot rifles with broom handles and that's the same idea (vertical grip) but dependent on the rifle of course.
When zeroing, you could also rest the rifle on the magazine, and put sandbags under the stock to get a steady platform. A correctly made lower and magazine shouldn't have any issues shooting like that.
Holding it by the handguard isn't that big of an issue either, since most people tend to hold the rifle much closer to the receiver than all the way at the end of the handguards.
And of course the point someone else made, at 25 yards, it makes almost no difference. I just thought some might find it interesting, as until I observed this phenomenon, I probably wouldn't have believed it myself, how big of a shift in zero it causes.