Having built over 20 wheels over the years I find the weld joint has little effect on the wheel overall as it is over such a short distance. Once you get a tyre on the rim, rotate it and see if the weld has any effect on the tyre shape and I bet you will not be able to see change in the tyre.
I usually get rims to less than 15 thou in all directions (I am an engineer) but it can take some time to achieve that and I have sometimes left a wheel and gone back to it the following day. As Peter says, up & down is more important so you don't want an oval shaped wheel.
I have found no difference between truing a stainless rim or chromed steel rim, but I have found old rims are the hardest to do. I don't know if they take a particular set over the years.
Simon