I have Big Foot levelers on my Class C motorhome and love them. But it is possible to bend the pistons. I happened to me when I tried to jack up the side of the motorhome to work on one of the rear mud guards. Jacking the left front and left rear jacks to their highest positions caused the rear jack piston to bend which necessitated it's replacement at a cost of about $450. a painful lesson in "what not to do". I could have accomplished the same thing by extending both rear jacks to their maximum position and I'm quite sure that no damage would have resulted from that maneuver.
In any case, to respond to the OP, using blocks of some kind under the jacks would have likely prevented the piston from bending as I would not have needed to extend it so far to get the clearance I needed to work under the rear of the motorhome.