all of my supports (at this time) are loose, and i have a cross cut set up to nibble away at as i need to shorten , hard to eliminate dips, but like you said cardboard under track does the job, i also use cardboard under entire layout, twofold, one it is about right scale for ballast, and it quiets the whole thing, i use standard cardboard and box cutter right up to edge of ties. once that is done paper mache and that is done,,, then smooth in the structure for the terrain, then plaster it so it has some strength.
here is the layout plan
