I'm constantly switching out tires for cross races, long road rides, and training rides on crushed limestone. For the crushed limestone rides, out of sheer laziness, I only change out the front tire to a 32mm knobby, and leave my 26mm slick tire on the back. The knobby in front gives me a little better handling on the limestone and gravel, and it seems to work pretty well. Is there any reason not to ride on mismatched tires? Does anyone else do frequent tire swaps like this? Are wheel swaps preferred?