I can see the logic in fitting the bars with weight on the wheels, because this is the static stress position of the frame.
However, drilling the chassis so accurately, which is easier from below than above, where the reaction bars are attached complicates this.
So I don't think it matters when you attach the top of the bar to the upper kingpin bolt. But take care because that fine thread can be difficult to restart if there's the slightest shift in the kingpin. With that bolt not quite fully tight, you can swing the bottom of the reaction bar into place on the chassis, and refit the wheel to check clearance. If necessary, file the inside corner of the BRB so the end fits really tight against the vertical part of the chassis.
Measure where the hole needs to be as accurately as possible with the weight on the wheels, then remove the wheels to get better access for drilling.
I've done this job once, and did it all with the wheels off.
