Every jelly roll I have has selvedges on the ends of the strips. They are cut across the width of the fabric from selvedge to selvedge. Not all fabrics from the same manufacturer are exactly the same width so the length of the strips may vary some. Plus not all selvedges are the same. Some are wider than others, some are white and some are printed the same as the fabric. Even the end with the selvedge the same as the fabric should be trimmed off.
You probably shouldn't figure on a useable width more than 40 inches.