I know. Sounds counter-intuitive, doesn't it?
Maybe I should rephrase, however.
Never begin knitting the second sleeve when you are more tense than when you knitted the first sleeve. Because you are likely to find yourself more tightly, and then when you measure it turns out you're getting a much firmer tension than you got on sleeve 1 (20 stitches to 4 inches as opposed to 18 stitches), and you are then going to need to pull out everything you've done on the second sleeve.
Yeah. Ask me how I know.