On my 6PPC, if you don't turn onto the shoulder the full width of the neck turner cutter chamfer, bolt closure requires a great deal of force, and if this is done (with the lugs well greased, to prevent their being galled) a so called "doughnut" ,at the base of the neck, is formed.
In my opinion, with the short bullets that we use in short range Benchrest, and the usual throat configurations, doughnuts are more of an aesthetic problem than a functional one. On the other hand, hard bolt closure risks lug finish damage, and should be avoided.
When I first started turning necks for my PPC, I was concerned that turning on the shoulder would weaken the case, so I sectioned a couple using a reinforced abrasive wheel on my cordless Dremel. (Wear glasses and a paper mask for this, since the brass dust is thrown right into your face.)From that, I could see that there was no problem. I should add that I like an old Sinclair turner for my rough cut. It has a rather broad chamfer that is easy to see which makes controlling the cut easier. I strive for an end of cut, on the shoulder that does not easily catch my thumb nail. A little trial and error has taught me that if I reverse toward the mouth of the case just as soon as the cut reached the full width of the chamfer, I should be OK.
Setting the case fit by the cut on the shoulder might not be practical if your chamber is very short, but for mine it works fine, and I don't get doughnuts.