new T bar

From what I have read, the reason they are saying that is that you don't want to tighten things up with the suspension drooped, then put weight on it, because it can damage the strut rod and lower control arm bushings. Tightening in the unloaded position, then dropping the weight on them puts a lot of stress on the bushings that can't move. You want everything in its normal road position and tighten them there, so they are centered in the travel range and not overly twisting those bushings.

Same thing for adjusting ride height. I'd imagine it's a good idea to loosen those before making major adjustments, then tighten them back up.

If it were me, I would just set it on it's wheels and snug them up to keep everything in place for now, because as you said, you will be resetting everything once everything is complete anyway.