I do alot of them and like alot of areas the rockers being one of them the factory mig welded along with spot welds. You see it alot at the sail panel (roof to 1/4) rear floor pan centers just to name a few. I spot weld then seam weld long beads just to be over kill at the rocker and it can be without solid welds as long as you weld around 75% of the seam . I never had problems with body work cracking and have done them like that with no issues for years. See sheet metal being undrill and re-welded on my video's section of my web site www.Jaycuda.com I put alot of cars in metal work on the site.