Yes, you can re-use the stockers. If you have the extra money, why not invest in some stage 8 locking bolts? They're guaranteed not to back out.
If you re-use the stockers, I suggest bolting the new headers back on, warming up the motor to operating temperature, letting it cool, and getting it back up to operating temperatue and re-tightening the bolts. Or some anti-seize on the ends of the bolts may provide cheap insurance that they won't back out.