It depends. (I always love that answer )
For things like benchrest competition it really doesn't matter what the case volume is, as long as all the cases are the same. 'We' have had discussions over the best ways to measure volume but the easiest is to weigh the cases. Do a full length resize or fireformed (pick one, another long discussion) and trim to same length. Weigh as close as you want. Pick 10 cases that weigh the same and use those for competing/groups. You can also use a fine substance, like sand, to fill the cases and weigh that. Some have used water, but, that is 1) messy and 2) not as accurate (IMHO).
For max loads it is more difficult. If you are not measuring the pressure you really don't know. Determining over pressure by stiff extraction, flattened primers, etc is a crap shoot. Note that you can be pretty far over pressure before seeing signs of it. I just use same headstamp but I am also a little below book max.
Short story. A customer (not mine, gunsmith was a friend) brought in a Dan Wesson in .357 cause it wasn't shooting well anymore. Test fire confirmed bullets were keyholing. Looking closer the factory rounds were 'loose' in the chamber, as it rattling around. Customer admitted to shooting reloads and had cases. Primers were not flattened out. Cases were not stiff to extract. When asked the customer said the load was Bullseye. When asked how much he said he just filled the case to not leave an air gap. He was lucky.