It's a PROBLEM!
Regardless of whether or not every Mopar that left the factory does it or not, its still a problem!
And I can tell you right now that if I went out and did an emergency stop from cold that I WOULD NOT end up sideways. I know this because I've done it, ended up sideways, and went back and fixed the problem.
Most of us spend a lot more time figuring out engine upgrades, heck, even transmission upgrades, than brakes. But your brakes are the most important part on the car.
If your car doesn't stop right during an emergency stop- FIX IT.
Smaller rear wheel cylinders will help. Making sure you're running the same material on all your brakes (semi-metallic is best) is important too. And if that still doesn't fix it, then add that proportioning valve. Assuming, of course, that all the brake hardware is in good working order.
As I mentioned already, I run the later Mopar 11.75" front disks and 11x2.5" rear drums on my Challenger, and will on all of my A-bodies as soon as they're on the street (already have all the parts!). The braking force seems to be much better balanced "out of the box" than the 10.98" rotors and 10x2.5" drums. I still run the larger 15/16ths rear wheel cylinders, but with a 15/16th's master cylinder, full stainless lines, and all new hardware, pads and shoes. I also run drilled and slotted front disks. Tires make a big difference too, if you're running a staggered set up that can make a difference, as well as tire choice. The 275/40/17's I run all the way around make my brakes work for their money.
Regardless of your set up, spend the time to get it dialed in right. Brakes are one of those things that should only be stock on a museum piece. If you drive it, you should be able to stop it.