Man, just like anyone else would tell you, over time headbolts can stretch or fatigue. So generally when rebuilding or swapping, whatever, you should replace them because it'd be hard to identify if they have warped or fatigued at all.

I have re-used headbolts on stock motors that didn't have a lot of miles or abuse on them, but I compared them to fresh bolts and cleaned them up pretty good first.

They dont cost that much brand new so its one of those things you shouldn't skimp on and just get em new.