Lap welds are much more common that butt welds. I know of no car that used butt welds on panels in production. The warpage of panels WILL occur on large sheets of steel. That link of the patch was best case, small patch, with compound curves and close to supports.
If the butt weld is not going to be seen then a backing strip is common... but most will flange and weld. As long as there is primer/paint you should be fine.
