What I always do when renewing brake pads,is,remove the old pads and clean the calipers with brake cleaning spray.Then carefully pump out each piston until I can see a clean part emerge,using something like a piece of wood to stop the other piston coming out.Then using a twisted rag soaked in brake cleaner loop it over the protruding piston and clean it.Then using another twisted piece of rag with rubber grease on it, smear a thin layer of the rubber grease all around the piston.
Then push the freshly cleaned and lubed piston back in and the do the same with the other piston making sure the freshly cleaned one stays in when you pump the second piston out.I served an apprenticeship with a brake and clutch specialist in the sixties and we always used rubber grease to assemble master and wheel cylinders after overhaul.
Bonne, I would use a little high temperature grease on the back of the pads if even if there were anti-squeal shims fitted.It just stops the ends of the pistons corroding.