[tireburner163]

Rear springs should be a little stiffer for a drag car but many people think that the car should "squat" during the launch so they install softer rear springs on the car to accomplish this. The only cars that should be squatting during the launch are those cars that shock the tires very hard (5-speed cars) and with enough power where the car will overpower the tires. Prostock cars actually squat during launch because they develop enough torque and hit the tires so hard that the squatting helps absorb some of the shock and keeps from overpowering the tires. But, setting up the car to squat is not a function of the springs. It is the position of the Instant Center (IC) that determines how the car squats. The STOCK mounting points for the control arms already cause the car to squat which can unload the tires. So in order to compensate for this squatting the springs need to be stiff to transfer the force to the tires.
If the rear end squats that means that the rear springs/shocks are not pushing the rear tires into the ground. They are just compressing. For the most part you don't want the rear to squat because you want some resistance from the springs/shocks to transfer the force/weight to the tires.

Example:
Make a coil spring out of a coat hanger and place it on a bathroom scale. Push on the spring. The scale is not going to read much because you are not transferring any force to the scale through the softer spring. Now take the spring out of your car and place it on the scale and do the same thing. The scale will read more pressure because it IS transferring more force through the spring.