the middle chain ring and various "middle" cogs are compatable, correct? Or are you not supposed to use this combination of gears?

All I know as of now is that "even on 5000 dollar bikes" (according to the lbs guy) you are not supposed to use the small chain ring with the 3 smallest cogs.....that's about it...any more precautions I should know about? Is this precaution even true, or is this guy fooling me?