Sometimes just pry it back with a screwdriver between rotor and pad, if it slides real easy.Yes, you are correct, but not absolutely necessary, just to make your life easier.
On a two piston caliper, it is absolutely necessary.
But don't use plywood, use the old brake pad, it is already there anyways.
Nothing wrong with a nice painted up, made to order, piece of plywood, or wood lath, though. Some like to work with wood.