It's a fairly common practice on the older vehicles at most body shops to apply heat to a rail, but they don't heat it up to a red glow. Most of the time the bend or kink can be hammered out while the frame is being pulled.
Like on our old trucks it's probably not as big an issue as the newer Hydraform frames on the GM & Ford trucks. Toyota recommends that no heat be applied when pulling their frames, and on these new frames for GM, its 90 seconds for 1500 degrees, I believe, You can check
www.i-car.com and go to the technical section and it will tell you all about full frame vehicles and what is recommended.
Now as far as I'm concerned all the bag jobs I have done, where welding has been done on the frame, I have not seen any cracks or weak spots. Maybe I have been lucky. I think common sense prevails when you weld, don't get it cheery red in to big of an area.