As others have posted, the key to cutting straight strips is having your ruler *exactly* perpendicular to the fold. To help understand this, fold some pieces of typing paper and then use your ruler and rotary cutter to cut strips. Try cutting a strip with the ruler slightly tilted from the fold, then unfold the paper. You will find the "V" shape at the fold. Then try cutting a strip *making sure* that the ruler is exactly perpendicular (90 degrees) from the fold. To do this, you make sure that the fold is exactly on a line in your ruler. It helps to use two rulers, so one is lined up with even more of the fold; your cutting ruler should be butted up against the other one, and a line on your cutting ruler should also be on the fold. Your resulting strip should have no "V" at the fold.
The further off your ruler is from the fold, the bigger the "V" will be in your strips. Varying your cuts on paper will show you how this holds true.