zetec7 wrote:
You may find that the position of the seams effects the way the tubing warps during bending somewhat, but it's not a big enough problem to worry about. Make sure you CLAMP those frame tubes down to the build table, and try to alternate sides & ends of tubes when tacking and welding. That will keep warpage to a minimum.
I was really OCD about this during my frame construction, and it helped a lot. My frame is within 1/16" or better on all its diagonals. It doesn't really make a big difference in the frame's strength, but it does make adding parts later easier if the frame is as consistent as possible from side to side, etc. (i.e., it's nice to be able to make 2 identical parts, or identical mirror-image parts, knowing in advance that they'll both fit the car precisely).
I'll add this to Zetec7's comments, don't remove the clamps until the welds have cooled to room temperature. Rushing here will cause no end of headaches down the line.