James,
Some people would tell you that 200 pounds per square foot are required.
We have good results with 50 lb/ft2 in our lab kiln. This is about 4" of concrete.
Everything else should be done well before using weight. Stickers every two feet, stickers aligned vertically, bunk above/below stickers, and the kiln cart supports located directly under the stickers.
Just having something over the pile probably helps keep the air flowing through the unit. Most kiln tracks are baffled on one side only. In a unit where the boards are not tight sided-to-side, air can move between the board edges up to the next sticker slot or up above the load (like having an unclosed chimney). The weights probably help reduce this too.
There are a few mills in the PNW and more in the SE using weights, maybe someone from one of the mills has something to add?
Mike M.