you need to make sure the ruler is lined up with the fold. One teacher recommends you realign about every 6" of cut.
As long as your ruler is lined up with the fold you will have straight cuts. If you are cutting 2.5" strips then re adjust about every 2 or 3 cuts and straighten the side if you need to before making the next cut. Your fold can be crooked but as long as the ruler is straight with the fold you will get straight cuts.