I use the following 9 gears out of a possible 21
Front-Rear When used
Low-1 Going up my driveway!
Low-2 Rarely
Low-3 Very steep hills
Med-3 Mod steep hills
Med-4 Rarely
Med-5 Slight inclines
High-5 Slight inclines, Head wind
High-6 Head wind
High-7 Flat

This is based on Sheldon Brown's recommendation to use gears that stretch the chain the least i.e. don't have chains in extreme diagonal positions