Mike - FWIW, I used 2x4 legs and 1x4's for cross-members on my new layout which is basically ladder-type construction. I also used 1/2" plywood sanded on one side and built on 16" centers for the cross-members. The layout is extremely solid - even with 1" foam on top - I can crawl on it no problem.
Assuming you can keep the cuts straight, I see no problem with using ripped 2x6's for the legs.
Like Mary said, I used nails only to hold the wood in place in some spots before securing with 2" drywall screws for the frame and 1 1/2" for the top.
I also drilled holes for wiring in the cross-members with a drill press before assembly.