I think that depends on the material being welded. Thicker material, where you can get good penetration withoug large gaps in the metals, the butt joint is strong. For body work, where the metal is too thin to bevel to get penetration without burning through or having to grind off the weld it is a different story. ICAR, the organization that sets the standards for Auto Body Repair, recommends a "backer" behind any butt joint. They recommend either a lap joint or butt joint with a "backer" that is plug welded on place for any patches. A lap joint done properly should not need an axcessive amount of "bondo" to complete the job, and will be stronger.