Torque To Yield (TTY) bolts should not be reused. A TTY bolt stretches when you tighten the bolt to the specified torque. You tighten the bolt so much that it goes over the yield strength of the bolt. Anything up to that specific yield strength, and the bolt will not stretch and will go back to original length. But once you tighten the bolt to where it is over the yield strength, it will stretch and not go back.
Well, so what the bolt is a little longer? Well, the problem is that when you reach the yield strength and the bolt starts to stretch, if you release the tension and try to tighten the bolt again to the same torque, you can end up breaking the bolt. Once the yield strength is reached and the bolt stretches, it will be much weaker. If you try to reuse head bolts, they can break on you very easily.
With that said, I have reused many TTY bolts. You just have to know how tight is tight and don't overtighten them. If you feel the bolt starting to stretch and not getting tighter, stop. Back the bolt out and get another one to replace it. You really have to have a feel sometimes when reusing TTY bolts. Trust me, having to pull your head and try to extract a broken bolt from your block is not fun.
