i think you are partly right but maybe for the wrong reasons. I let my car warm up too but only so the oil gets hotter and thinner and completely covers the internal parts before i go driving. I don't think it matters about your parts expanding and what rate that happens at. It's good that you think about these things though. The theory behind my warm up period is I want the oil that has drained to the bottom of my oil pan to get up to the cylinder head and piston rings, coat the parts before i go. thats all