I haven't done this myself, but I have heard it's best to do it when the weight of the vehicle is actually on the tires. I'm sure others will comment when the sun comes up on the northern hemisphere.
to me, I`d rather have the car setting leveled on jackstands. most of these cars have taken a twist after setting on springs that don't age and settle the same, or are twisted from bigger h.p. I like getting it as square and level as possible, then welding in the subframe conn. even if you have to readjust the doors a little. if you do, it proves - it wasn't setting right to start with. why weld one up that`s crooked ? if ur planning on big h.p., u might want at least a 6 point cage and or rocker boxes.