While what you say is true, it is not great advice IMO.
Better to remove the calipers from the rotors, remove the pads, fill the reservoir, push the pistons FURTHER OUT, then clean them to a smooth bright finish with brake solvent and a shoelace. 600 grit sandpaper if they are really bad. Once the exposed pistons are bright, drip some brake fluid onto them and press them back fully into the bores (which I do with the bleeder open to purge old fluid), etc., etc.
My point is that pushing dirty pistons back into their bore does no good whatsoever, any way you look at it.
I go further and recommend to suck each reservoir dry (small baster with tubing), clean the spooge hole and reservoir well, then refill and as you push fresh fluid through the open bleeders, repeat the baster suck-out and replenish with fresh fluid periodically. You can see that each pressure stroke/release flows "dirty" fluid back into your "clean" reservoir. Really, a quart of fluid is so cheap, try to use it all!
I admit, being new to my 04 ST1300, that I have not done my brakes yet (although I did replace the front tire and cleaned the front pistons - the pads had mega-meat left at 18,800 miles) the idea is universal for all calipers. Push the pistons further out, clean them, lube, retract, wipe the excess, re-pad. No sense in pushing cruddy pistons in and compromising the seal's life.