I've done it both ways ... but the headspace guages definately give the home hobby barrel swapper a 'crisper' feel when the barrel touches. I find cases feel 'dead' when the barrel screws on and touches the case, so much so that when I had my 223Rem barrel rechambered to 22-204, I headspaced on a fired 204 case, but then took it to a gunsmith to re-check using the headspace guage. My settings were fine, but that 'dead' feel caused me to question my work.
Cheers...
Con