Tony of Kahntrol.com posted a clever method for leveling a reticle in a scope. He goes on to explain that due to errors in assembly the reticle could be slightly off. There could be errors in the machining of the scope body for those who use scope levelers like the Spuhr or Arisaka leveling kits.
Tony’s method is to use a white background and a plumb line. He taped a white piece of paper and hung a plumb line in front of it. He puts the butt of the stock in front of the paper and as close to the plumb as possible without touching it. Then he shines a bright flashlight into the objective of the scope. He focuses the eyepiece to show a crisp reticle shadow on the white piece of paper. Now he can adjust the scope to get the reticle to be parallel with the plumb line.
Go here to read his article.
It is such an easy way to make sure the reticle is straight. I think I will go check my scope now.