Kathy,
I haven't read what others have said, but I have a tip for perfectly straight strips every time. The ruler is not the issue, other than to cut across a 42" or greater fabric, you need the long ruler, 24". Here are the steps.
1. Always cut on the single fold, not double folded. In other words, when you lay your width on the mat, you need a large enough mat to lay it out as it comes, only folded once. When you double fold the fabric (fold it another time to have a smaller width), it is nearly impossible to get straight cuts, no matter how straight it may look.
2. Only cut 2 strips, then do a straightening of the fabric before cutting the next strip. You do this by lining up the fabric on your mat using both the horizontal and the vertical lines. You will find that when you line it up
with the horizontal lines, there will be a sliver of fabric that is over the vertical line. Lay your ruler on that vertical line and cut off the sliver. Then cut 2 more strips and repeat the process. If you do these simple things,
your will have perfectly straight strips every time, no matter how many strips you have cut.
I find that with cutting the border strips, especially for the long side, if you don't want to piece it, measure the width, say 6", then put a snip at the 6" mark, lengthwise the fabric, and tear it to the desired length. I know a lot of people cringe at tearing the fabric, but if you do that for a long length, it will tear straight and your borders
will not be wavy. If you prefer to piece it, cut as many strips as you need to end up with the desired length for the borders. Then, after you piece them together and before you sew them on the quilt, measure the middle width of the quilt, the top measurement, and the bottom measurement. Then take an average of the 3. For instance, if the middle is 45", the top is 46" and the bottom is 46", cut your border strips, 45 1/2". Pin those on the top and bottom, working in the excess on the top and bottom as you pin. After sewing the top and bottom, repeat for the sides, measuring vertically. Pin and sew. If you do this, you will have perfectly straight borders - no waves.
I realize this may sound confusing. If you have any questions, pm me and I will be happy to try to explain better.
I can promise you these simple cutting suggestions, and then measuring for the borders, will yield a perfectly straight quilt every time, and you can wave goodbye to waves forever.
Marcia