Re-using is generally OK as long as they are not the torque to yield type used on some newer engines. Re-using torque to yield is a no-no because stretching them the first time changes their properties and a second stretch may overload the bolt and joint.

For standard head bolts, a thread guage is a good way to check for stretch. I was working with an experienced mechanic on a 60+ year old flathead. Those bolts had been re-used who knows how many times. He checked each one before re-installing and had to use some spares to replace those that had stretched in the threads.