I did it on my tank. I thought that it would have two benefits. One to spread out the weight of the rock, and two to stop the rock from sliding on the smooth glass bottom. I have no idea if this actually helps or not but for the $10.00 in egg crate I figured why not.
I found an even better solution that is a bit more time intensive. I saw a picture of a guy's build on the net who did it and thought it was brilliant.
I took 2 pices of 1/4" acrylic about 1' x 2' and drilled 3/8" holes in a grid pattern every 3". I then took 1" pieces of 3/9" acrylic rod, drilled them with a 3/8" bit until one tip was indented to the edge and then ground 2 grooves into the top so it had a sort-of tulip pattern (for gripping --- you could just do flat cuts I would imagine). These were then solvent welded into all the grid holes and then laid on the bottom of the tank. The LR is securely held in place by the rods but organisms can still move freely through the sand bed but without destabilizing the LR.
I used eggcrate and covered it with approx. 4" of sand. I wanted to protect the bottom from direct contact with sharp edges and points of the LR should it settle or be undermined by burrowing inhabitants. Worked well for me.
I like jtma508's suggestion with the acrylic sheets and rods. I would glue the acrylic sheets and rods together first. Then you could use a drill press to drill one hole through each rod w/sheets instead of three separate sets of holes.