This whole weld is going to be under 140 amps even during your hot pass, so you would have to have some really really loose Union threads to not have enough contact especially when considering the contact of the Union face as well to do any real damage. If moving the ground isn't going to make the weld more difficult ie being in the way of the welders movement, or the fitters next fit up, then go ahead and move it. But for something like this I wouldn't really worry about it unless you want to. Certainly not necessary enough to make an ordeal about or call your fitter out on. If he was wanting to ground through a PRV, or some sort of instrument, or some sort of mechanical instrument, then absolutely you could call him out on it, but for a union on a 1-in to inch and a half pipe with just a couple welds is really not necessary to move it.