I suppose time will tell on this one, but every bottlenecked cartridge I load for, I make headspace on the shoulder, and that includes rimmed, rimless, semi-rimmed, rebated and belted mags.
When I shot service rifle with a SMLE, it was not at all hard to make a 303 headspace on the shoulder.
I can even get my 450 Ackley to headspace on the shoulder, and it's got next to no shoulder. Even old Elmer Keith never had a drama with his 400 Whelen.
I even necked up 30-06 to 358 and back down to 338 for my 338-06, just to get the crush fit when the bolt was closed.
In reality, the belt on belted mags, for me, is just there for looks and serves no real purpose at all.
From my experience on this one, I've got to agree with Daryl.

DC