I made mine with a slight tilt. I didn't measure the angle but I'd guess it's around 6 - 8 degrees. The planes will sit on it just held by gravity, but I put the whole thing on casters and I was concerned that the jostling might cause them to fall so I made cleats for the heels and toes.
"History is strewn with the wrecks of nations which have gained a little progressiveness at the cost of a great deal of hard manliness, and have thus prepared themselves for destruction as soon as the movements of the world gave a chance for it." -Walter Bagehot