Not sure about Cutlasses, but since the Regals had both front and back aluminum bumper cores, it make sense that the Cutlass would too. Maybe if there is a car that has one steel and one aluminum, it was in an accident and the body shop ordered the wrong one. I think the aluminum ones are better since I have yet to see a steel one that is usable, plus they are much lighter. The only drawback is that you have to be careful about the steel touching the aluminum, which can be very bad (electrolysis, the dissimmilar metals eat each other away). I think the factory used a thin plastic film on the mating surfaces, which seemed to be effective.
There is also the aluminum radiator supports. These seem to be on random cars from the factory too. I think the only cars that had all/most of the aluminum parts were the diesel Cutlasses. But then again I'm not sure because I have never seen one in person.
I think all Monte Carlos had aluminum bumper reinforcements. I can't really say for sure but all the ones I have seen torn apart at salvage yards, regular, LS, SS, had aluminum.