It is the *angle* of the ruler respective to the *fold* that determines whether or not you get the dreaded "V" in a strip.
To test this, fold a piece of paper. Cut a strip where you try to get a perfect 90 -degree angle of the ruler to the fold. Then move the ruler so it is either more or less than 90 degrees and cut another strip. The further away from 90 degrees the angle is to the fold, the deeper the "V" is in the strip.
Consider the edges of the paper to be selvedges. It doesn't matter whether the selvedges match up or not. Fold a piece of paper so the edges are whacky. Then cut a strip with the ruler exactly 90 degrees from the fold, wherever the fold is. The strip will come out without the dreaded "V" (edges will be wonky). Likewise, grain of fabric is irrelevant. Straight strips can be cut from fabric that has been folded on the bias.
The easiest way to achieve a perfect 90-degree angle, in my opinion, is to use a June Taylor shapecut. As long as you ensure that a line on the shapecut is exactly parallel to the fold of the fabric, you will get 90-degree cuts.
Another method is to use two rulers to ensure degree accuracy. The first ruler (the bigger the ruler, the better) is placed so one of its lines is exact with the fold of the fabric. The second ruler, which is the cutting ruler, is butted up against the first ruler. You then also check that a line on the cutting ruler is exactly parallel to the fold.