The biggest risk is warping the axle tubes. Many have successfully welded theirs by starting at 12 o'clock and welding about an inch. Then cool and weld an inch at 6 o'clock,cool, then to 9 o'clock, cool and then 3 o'clock, cool and then go back following the same pattern starting at 1:30, etc. Some use a jig to be sure they're 100 percent straight.
Dont do it your self, you need to put it on a jig and make sure the tubes have not already twisted. If you weld the tubes and they are indeed twisted then you will just create bind.
If you are going to do it make sure theres no weight on it. I lay a few beads around 180 degrees from each other, then 90, then fill up the rest. It may also help to slow cooling down by placing rags over it. Why do you think your going to spin the tubes?
IMHO, welding your axle tubes is more trouble then its worth unless you have a buttload of horsepower and sticky tires. in otherwords you drag race. Now if your gonna weld your axle tubes, you also should have upgraded the axles and diff first. ive really never head of axles tubes going in most street performance 5.0s but then again im not immersed in high powered 5.0s all the time. all i can speak for personally is my own car.
I put a 1 in bead on top and on bottom of my 8.8 housing.
I did this with the theory that doing opposite sides of the tube should minimize any bending. I was just too lazy to try to weld all around. I wanted just enough to help resist the tubes twisting in the center, and cracking the center casting or overwhelming the Ford plug welds. May go all around next time I work on it.
Axles will tolerate more misalignment than you think. Stock car guys set up rears with the tubes bent to get camber in the rear.