Very true (pardon the pun). One would not expect this from a custom precision build but take something like a Savage 110 for example, where the taps are off (ever so slightly) on that round top receiver. Then, your rail and or rings are off too. No big deal if it's equal front to back but it can be a pain.I have one of the Arisaka tools, but when the base or rail itself isn't plumb and true on the receiver you end up going a ways down the path before discovering that you have a problem.
The other thing with smaller sporting rifles is that you think you have everything true and plumb (and it may be) then you shoulder the rifle and depending upon your cheek weld, you think you're standing on a roof.