From a pure materials standpoint, once the steel of your vehicle was hit the mechanical properties in the deformed areas were changed (crystalline structure, etc). Now, those same areas will be *bent* back into place causing increased mechanical stresses in the material (read: weakened). It's like the flip-ring on a soda can, bend it once (to open the can) and it's fine. Bend it a couple more times (i.e. - work-harden the material) and it'll break. Same principle applies to your car. Once the structural integrity has been breached, it'll never be the same (or as safe) as the day it rolled off the assembly line. Sure, it may not "look" any different after the body shop's done, but it won't provide the same amount of safety as before. I'm not a body shop expert, and they may do some heat treatment to relieve some of the induced stresses, but face it YOU'LL know your car isn't the same as before.
Someone with body shop expertise, please check my statements
Just my 2-cents.