I was mounting my new tires the other day, and just as I finished I noticed a tiny little arrow molded into the sidewall... D'OH. Luckily I had unknowly already mounted BOTH the correct direction.

I realize i could fix a flat without completely removing the tire, but honestly, it's the last 6 inches of tire to go on that is the trouble, the rest is easy, so I just take it completely off for ease of fixing.