It's worth mentioning that the spring rates of RWD cars differ way more than of cars with other drive train layouts. As you might already expect after reading the previous part, opposite spring rates could be quite handy for 1/4mile tuning.
With some cars, but definitely not all, soft/stiff is indeed the best for a good launch, although you'll often lose a bit at the end. But this effect isn't as crucial for 1/4mile as it is for longer distances.
Rear - using opposite spring rates
The logic behind a stiff rear spring rate is possibly the positive effect on the ride height. It'll keep your rear high, which is a good thing for maximising traction.
Front - using opposite spring rates
Now this seems to be confusing at first, cause the front will raise more than with stiff springs. But you must consider that there's also a positive effect as result of it - weight transfer. Now, it may be a good idea too quickly hop to the weight transfer part of this guide.
Weight transfer would be actually a good thing if it just wouldn't hurt ride height, because, as I mentioned in the weight transfer chapter, in GT5 ride height is the most important thing suspension-wise. But if you're going to use superstiff rear springs anyway, this could limit the height change well enough so that the positive weight tranfer effect outweighs the negative effect of it.
Beside of this as a nice side effect it even lowers the car's ability to bog and let's you therefore set a better gear ratio.
Be also aware that due to the soft springs the damper setting could gain importancy, but simply read the damper part of this guide for this.