i was always told to match - but it's a little hard since i'm darker where the sun hits (i.e., forehead, nose).
sometimes you need 2 - one for summer when you're tanned, and one for winter when you're pale.
i only buy one since i only wear powder on occasion. (foundation, i buy one but the MA was nice and made me a sample size of the next darker shade so i can mix it when i get a little more sun)