OA warps worst, TIG is not so bad, MIG the least. Warpage or distortion is a product of how much time you spend on the panel with the energy source. Consequently, the OA is easiest to work out even though there is more, then TIG, and finally MIG being more difficult because of the weld being "harder", what happens is the molten pool supercools allowing a too large pearlite in the grain structure. More difficult, but not impossible. I am talking but welds too, all with the same zero gap. I chose my method by what machine is not being used by someone else, what machine doesnt have an empty bottle, the cool factor, and by how much I do or not want to grind.