It'll be better to then to choose springs with the same rate front and back and fine tune with some adjustable shocks? or a set of coilovers i assume?
Tein has thrown some time and effort into engineering their products, which is why they are so popular. Now, they've designed their Flex kit for the 350Z with 12kg/mm springs front and rear. If you put 12kg springs with the stock shocks, the thing would understeer like a pig before skipping off the road due to the lack of suspension compliance.
The Cusco Zero-2 coilover kit comes with 10kg/mm springs up front, and 7kg/mm springs in the rear. So, they've gone even further and made the back softer instead of the front! The complete opposite to stock.
But, with the engineering work they've done, Tein and Cusco have come up with valving inside their shocks that match the spring rates they've chosen, and the 350Z's balance. You get some adjustability to fine-tune the balance, but it has to be near enough to suited and the range of adjustability will be within the spring rates' tolerances.
I drove cars equipped with either back to back (thanks MR RIZK and mchapman) over a section of roadworks (where they'd cut parts of the tarmac back to the concrete all the way through an S-bend), and the 2 cars at 35mph felt almost identical driving through those massive craters. I was only testing ride, so I have no idea what the 2 cars are like when at the limit, but on spring rates alone they shouldn't ride the same or the way they do if you compare it to the approx. 5.0kg/mm springs in the stock setup.
Now if you buy shocks and springs separately, they'll come with certain valving inside. But, what springs did the shock manufacturer work with when they designed the product? What were the rates? Was the roll stiffness compared to stock modified? Did they use aftermarket swaybars? Were they just stock springs? If you can't ask them, you have to figure it out yourself. Hence why you become your own suspension engineer.
With a kit from a reputable company you know they've spent the time and effort making sure all the parts work harmoneously. That saves you from having to do it. And you'd hope that they, as a big company, know more about suspension than you (some average guy) so they're more likely to come up with a good suspension setup.