Im pretty sure, as heavy as the rear housing construction is, that truss wouldnt pull that rear end unless you got it stupid hot.
Best bet, if you are concerned, is to stitch it at random points along the truss. Then randomly connect the stitches. That way you are never concentrating heat in any one spot. This is how I have always welded long welds to control warpage. I have welded many a truss and have never seen one pull an axle out of line.
(also...if you have ever had to straighten a bent axle tube, think about how much heat it took and even then it had to be quenched to cause the axle to pull very much!)
