The advice you got wasn't correct. At 40,000psi (or more) taper isn't going to make a difference at the time of firing. Pressure acts in all directions and there's more area (ie force) pushing the case against the chamber so it can't drive any harder against the bolt than what the pressure is doing anyway. What causes case stretching is excessive headspace.

How it works (it was explained to me on the ParrallaxBill forum, with animated images) is that if there is too much headspace, the firing pin will push the case forward in the chamber. When the powder ignites, the pressure forces the case against the chamber wall. The case head is solid and can't grip the chamber wall so it is pushed back against the bolt face - the case stretches where the solid head joins the case body. This is the part of the case where head separations can eventually occur.

What 500Nitro said is correct - if you neck size only, the rest of the case becomes the defacto headspacing surface. Normally rimmed and belted cases will headspace on the rim or belt, but you can make it headspace on the case shoulder on a bottleneck case, or even the case body in an otherwise straight walled (tapered) case. And you do this by neck sizing only.