your are allways supposed to change the bolts because they have a stretch limit to them so they wont break when the engine heats up from thermal expansion. Once you torque them down they arent exactly the same as new bolts and arent supposed to be used again, the factory specs of 56lbs tq only applies to new bolts and it may be less if you reuse them since they are stretched some already. The bolts should be at correct stretch and torque because if they stretch beyond their limit then they will either break or they wont fully come back down when the engine cools, the head will be lose then you have a blown gasket either way.
I didnt replace mine and they are fine so far, i've never heard of it causing a problem, but then again ive never heard of anyone else changing and head and using the same bolts. I say if you got the extra money its a good idea to protect your invest ment, i know you paid alot just for the job so you might as well buy the bolts.