Although I usually mounted my tires correctly I never thought it made much difference until a ride a couple years ago. I had Bontrager 700 x 35 Select K tires mounted on my Trek 520 touring bike. As it was winter I had fenders installed on the bike. Snow had been on the ground for 3-4 weeks previously but was gone when I started an 80 mile ride. The road shoulder was muddy with a real fine, silty mud. About 15 miles into the ride I had a three mile descent. When I got to the bottom the bike felt like the brakes were dragging. After checking it over I realized that the front tire was mounted backwards, and as a result mud built up and was impacted on the underside of the fender. I scraped off and then turned the wheel around. Since it was the front, of course, I didn't need to dismount/remount the tire. The ride was fine from then on with no more build up of mud. I haven't mounted a tire the wrong way since.