You should always replace the head bolts. Head bolts can and will stretch over time due to the fact they are usually steel and the head is aluminum, as the two materials have different thermal rates.
If you reuse the old bolt it could break, or it could bottom out in the threaded hole of the engine block causing damage to the block and/or not tightening down the head all the way.
For safety reasons head bolts should only be used once.
Do one thing every day that scares you. -Eleanor Roosevelt