Seriously though three main components ; bolt carrier, bolt , barrel extension are all critical and changing one of these can move the round forward on backward a .001" into the barrel.
The most common change would be swapping out a bolt and most people don't check after they have put a new bolt in.
AR-15 is more stardard, headspace gauges are not that expensive. Buy it onceand your good of life. 308 gauges cost a little more but same deal buy once. Or if your cheap like me , go to your freinds shop and put the go guage in then the no-go guage and hope for the best.
Here's a example your building an engine and you check the crankshaft to bearing clearance with a little piece of wax. You do it just so you know, doesn't mean you will remachine the parts but if its out of spec you know then a can do something to fix it.
Same with go-no-go gauges ,peace of mind check !