Patterning your gun is a very usefull excercise.
I took two other shooters that were having trouble hitting anything with their guns to the pattern board this past summer only to find out that their shot columns weren't hitting anywhere near where they thought they were! Both those shooters switched out their guns for different ones and immediately starting breaking clays!
I couldn't agree with Cat more when he said don't get too wrapped up in it though. Some guys like to pattern every choke tube they have and then buy new tubes cause they don't like the pattern, sometimes they gain a little ground, sometimes they loose a little ground by doing this. The thing is, every shot string is just a little different than the last one so no two are exactly the same anyway.
I've even seen guys switch out their tubes for custom jobs just because they didn't think they were machined to close enough tolerances. We're talking 1 thousands of an inch here..
. I've considered that these guys may be anal retentive loosers.
I usually try all my choke tubes on the pattern board just to check that they do hit where I expect them to for my own peace of mind, but I've personally never had one that hit in a signifigantly different place than the others.
Some guys think that resting the gun such as over a truck hood is helpfull but I've never seen any usefullness in this practice at all. Your only patterning at 40 yards or less so aiming isn't a factor. Besides, you don't aim a shotgun anyway, you point it! Just stand on your hind legs and point it at the center of the pattern board.