We know that the driveshaft torque tends to unload the right rear. It follows, then, that, if your suspension is symmetrical and your rear weights are equal when you're sitting in the pits, those rear weights are going to be very unequal during launch.
So, you have two choices: One is to put some asymmetry into your suspension (which I strongly recommend) or to statically preload the right rear.
If you're going to statically preload, weighing the car with suspension links disconnected is of no value. Think about it: When are you going to run the car with a link disconnected? Or, looking at it another way, why would you want the rears equally loaded statically, when you know that, as soon as you launch, the loadings are going to change?
But, weighing the car with it set up exactly as it will be at the strip is also of no value UNLESS you know how much preload you need.
Right about now, you might be wishing you had the money you spent on scales back in your pocket. Actually, those scales, when properly used, can give you an edge over most of your competition.
If you simulate...in your shop and without even starting the engine...the launch loads on your car, you can use those scales to determine the rear tire loading during launch. This is true whether you statically preload or use some form of asymmetry. You do this with what I call a "traction dyno." Its configuration and use is described at my site.