I've only done a few of the roof to drip rail welds.
It's no problem to weld them from underneath, but I'm gonna do the rest of them when the car is back on the rotisserie and upside down.
I had read that this (drilling the drip rail for welding to the roof skin) was recommended by AMD, and it makes sense, as there is more meat around the drip rail plug weld holes, and nothing to grind flat once you're done either.
Cheers!