I don't know if it's the "best place to camp," but, when I did this route, I stayed at Iceberg Lake rather than at UBSL. The next day I went over the pass and down to Guitar Lake. I had planned to camp there, but I arrived so early that I thought I'd push on and perhaps camp at the sites just below where the JMT meets the MMWT. But I got there earlier than I expected, too, so I hiked over Trail Crest and all the way out, making my trip a day shorter than I had planned.
Just as well. A strong snow storm hit the mountain, blanketing everything all the way down to the Portal. (This was in late September.) I read later that day hikers had stumbled down to Trail Camp, begging to be let into tents, because they didn't have proper clothing and because the trail was obliterated.
As for Arctic Lakes, I don't remember seeing appropriate camp sites there--at least not in the upper regions--but I do recall a fine one just over the crest of the pass. Had I known about it, I would have stayed there instead of at Iceberg.
