Are you talking about V bends in the fabric strips that you cut? If so, the issue involves how you are cutting in relation to the folds, and actually has nothing to do with matching selvedges.
To get a perfectly straight strip when you cut a folded fabric, the ruler *must* be positioned so your cut is exactly 90 degrees in relation to the fold. One way to do this is to use a T-square, where you line up the non-cutting edge exactly with your fold; the cut edge will then be an exact 90 degrees. The way I do it is to use two rulers. I line up one ruler so that it is exactly on the fold, then butt up my cutting ruler to the end of the positioning ruler. Obviously this is going to work better with one fold rather than two folds. With two folds, the folds must be *exactly* parallel to each other.
To test what I am saying, take a piece of paper and fold it in half. The fold represents the fold in fabric. Cut with a ruler so that the angle is obviously *not* 90 degrees from the fold. When you open up the paper, you will see a V where the fold was. The farther from 90 degrees the cut is, the wide the V will be.