The prop valve is needed when you have disc up front and drums in the rear. The drum brakes self energize so you need a valve to "blow off" the pressure. Otherwise the rear drums will always lock up during a hard stop.

With discs on all four corners you don't really need a prop valve since everything is linear. But you can get into a problem if you change the weight distribution or drive in different weather conditions.

Best thing to do is to set up the brakes so you're starting off with the correct proportions front to rear. Typically you want the rears to do 1/3 of the braking. So you set the rear up to have 1/2 the brake force of the front brakes. That rule of thumb works for most nose heavy muscle cars.