Porportioning valve needed?

May not be essential if the the front and rear disk setups came in the same kit from a company like Wilwood, all designed to work together, and your car is stock. Otherwise, smart to install an adjustable prop. valve and adjust the rears so they don't lock up until just after the fronts skid (test in wet parking lot). Even if your front & rear tires are different (wider rears, etc), it won't be stock.

Of course, if the rear disks are just for show (their main value) and your car will be a trailer queen and never drive faster than 10 mph at a show, no need for anything special.