For indexing especially, aligned drops are the first step, then, alignment of the derailleur hanger, that is vital in my view for happy shifting throughout.
It is possible to get by with just aligning the hanger, but why load the axle askew?
1 to 2 mm is probably tolerable.

I have the original Shimano hanger alignment tool set, which I always found funny suggested using a Campagnolo hub to be used in conjunction with it...not in the tech handouts, but in conversation with them when SIS first came out, they said a loose Campagnolo hub they knew would probably be at the disposal of most pro level shops...