The rear windows are made with smoked colored glass, like a beer bottle is. So they offer little to no UVA or UVB protection, little to no infrared or radiant heat transfer restriction, no shatter protection, and so on. The only advantage of factory "privacy" glass is a little theft deterrent and looks. So IMO your best bet would be to get *all* the windows tinted. (unless you like skin cancer, subpar air conditioning, a faded interior, glass in eyes, etc)
Here is my front windshield tinted.
My sides are dark as well:
Another angle of front: