You shouldn't use the big/big or the small/small chainring+cog combination. It's called "cross-chain" and it's hard on the drivetrain.
http://sheldonbrown.com/gears.html
"Try to avoid the gears that make the chain cross over at an extreme angle. These "criss-cross" gears are bad for the chain and sprockets. Especially bad is to combine the inside (small) front sprocket with the outside (small) rear sprocket. This noisy, inefficient gear causes the chain to wear out prematurely."