Perhaps that lack of stretching happens with belted cases, as rimmed cases will stretch at the web, if they are not supported by a shoulder, as will belted cases that are over sized in the shoulders when fl sizing. I am not saying the cases will separate upon fireforming, just that there is thinning at the web. I prefer to eliminate all stretching/thining.
I always neck up first, then down to form a head spacing shoulder before fireforming. Just something I do, automatically.
After fireforming, I only size the neck to the shoulder junction then check for feed and chambering.