If you are interested in cutting your fabric straight (especially for strips), use this tip to help you do so:
First, fold your fabric in half with the salvage edges together, as well as the already cut edges from the fabric store. Line up the edges that have already been cut and don't worry about the salvage edges.
Now fold the fabric again and lay it over the top of your seams so you don't get distracted by it.
Put your ruler over the fabric and line it up to make sure it is straight on either side. For example, in this picture I lined up the ruler at one inch on the top and half an inch on the bottom. No matter what, you'll want your fabric to be the same length from one side of the ruler to the other to show your fabric is straight. Go ahead and cut!