Should I flatten dimensional lumber?

I'd like to build the fold-flat workbench from the October 2011 issue (Issue 207). As a beginning woodworker, I'm curious: What is the logic behind planing the dimensional lumber in that project?
—Tom Walker, Boonville, Mo.

Along with cupping and warping, Tom, minor differences in thickness and width are common in dimensional lumber (2×4s, 2×6s, etc). Also, the rounded edges of most dimensional lumber make measuring and marking difficult.

To use these construction-grade materials in better-quality furniture and shop projects, you need to treat dimensional lumber as you would rough lumber. Follow the steps below: