Rob Johnstone: A true 3/4".

Chris Marshall: This digresses from the question a bit, but the issue of "typical" purchased 3/4-in. lumber not being a true 3/4-in. gives me reason to comment. If you have a surface planer and a jointer, don't settle for undersized lumber thickness. Buy your stock roughsawn. Often, 4/4 roughsawn will be a full 1-in. thick or just slightly less. It's surprisingly affordable, and you'll have some extra material thickness to help you achieve truly flat, truly 3/4-in. lumber. I know just where you're coming from in your question, and making that investment in a planer and jointer was one of the best improvements I ever made in my early years of woodworking. Taking control of your lumber thickness and being able to surface it accurately is a huge benefit! You'll never regret the purchase.