Welding steel under tension?????

[xpsled]

Im having issue with the chassis installation in my Falcon. I tacked the rails into the car and was very happy with the geometry. I then welded in a rear cross member only to notice that due to a lack of bracing the rail moved about 1/2" at the top of the wheel arch. I grabbed the bottle jack out of my engine crane and can very easily push the rail back to its correct position. However my question is should you weld when the steel is under tension or compression?? Will this reduce the strength of the weld/chassis by welding it when it when the steel is not in a neutral position? Any advice would be appreciated!!!!
Thanks
Dallas