If you want to set it up correctly I have posted the formula for calculating the desired spring rate.
You need your corner weights and the desired frequency in Hz, but there is also guides there on how to obtain them.
Stiffer rear springs will lean the car towards oversteer, but if you go too hard or have too little outward travel on the rear the risk of lift off oversteer increases drastically, and can be dangerous if you go overboard with it.
My opinion on springs is the car should be sprung to the desired frequency, and my opinion to suspension set up is as follows;
the front toe to should be set up according to the scrub radius, camber should be set up according to tyre temperature, and the tendency for over steer or understeer controlled by the rear toe. traction when braking or accelerating (also weight transfer, but they are directly related to one another) is adjusted by adjusting the shock bump and rebound.
Ofcourse not all cars have the ability to adjust all of these things, so you have to make adjustments elsewhere to achieve the desired affect.