A tip I've found worthwhile:

Use the same ruler for the entire project.

i just went down to actually compare some of my rulers.

I noticed that one edge of my Quilter's Rule 24 x 6.5 inch ruler is no longer straight and true.

It causes me great pain to discard a no longer "perfect" tool, but it probably isn't worth the aggravation of crooked cuts to keep it any longer.

I need to take a look at my rulers.

Not all rulers are the same. I have two different brands of a large straight. When I place them on top of each other I can see the one inch marks don't line up exact.

Now I buy only one brand of rulers.