I was in the shop last evening working on a shelving unit out of construction lumber (2x4s.) When I was securing the second stile to the panel I noticed that although I had measured where the rails SHOULD attach to make the shelves be the same height, that the actual board was warped away from that point by at least half an inch.
I popped a square on the corner and noticed that when forcing the board into the position that my measurement said the board should be attached, that the corner was very much out of square. If I instead fastened the board where it was resting naturally, it was almost perfectly square after tightening.
This led me to wonder, is it more important when building a shelving unit that my shelves be at the same height, or is it more important that the interface of the rails to the stiles is square? The plan is to screw in a sheet of MDF on each rail level.
I've added a sketchup I just cooked up to illustrate what I was building, please excuse the bad sketchup skills.