Checking Piston to Valve clearance

When checking Piston to Valve clearance is it best practice to use new stud bolts or the old ones? If using new studs is stud stretch a concern?

Most often when I'm building a performance engine I'll be using new studs for the build. Aftermarket stud kits are reusable and hence you don't need to replace them after a dummy assembly. Stretch isn't a concern assuming you're tightening them to the manufacturer's recommendations. Probably the only thing to watch out for is if you're using a factory torque to yield fastener which is a single-use bolt and hence shouldn't be reused.