I remember someone (forget where) mention that you should not fire the shutter with the lens already stopped down...I suppose this wears on the cameras aperture mechanism?
FD-lenses with the breech-loock ring typically allow one with the lens OFF the camera to set the aperture ring off from "A" and to arrest its automatic diaphragm lever at its end of way. One then can put the lens on the camera, by this stopping it down. In succession one can take photographs in the stopped down position. It will not harm the body, its automatic diaphragm actuator just will run into the empty.
BUT the lens now is stopped down and that will make the metering misfunction as the body does NOT know that the lens is stopped down. However the body kows that it should work in manual mode. (As long as the resp. body allows for manual mode.) So if you do not rely on the internal meter you can work with that setting.
If you push the stop-down button at the body after mounting such "cocked" lens, metering will be fine again. (But then the premature stopping down makes no sense anyway.)
This procedure will not work with new-FD lenses (with "true bayonet" mount).
If you mean mounting the lens normally and just then stopping down by means of the aperture ring and the stop down button at the body, there will be no issue. There will be no harm at the mechanics and the body knows that the lens is stopped down. Just keep in mind to meter correctly (as for the stopped down indicator at the AE-1)
With the automatic diaphragm lever of the lens pushed to the right (either by "cocking" it advance or by pushing the stop down button after mounting) the majority of that load is taken off the body mechanics, thus it not only would not harm the body but even relieve it.