Depending on your region and the exact location of your greenhouse, the cover will attract everything from moss and algae to leaves that inhibit light from shining through. No matter where you live, your greenhouse is going to get dirty.
The best times of the year for greenhouse cleaning are usually in spring and fall-before and after winter takes its toll. In springtime, you'll want to sanitize your greenhouse cover to rid it from the aftermath of winter; in fall, you'll want to clean off all the leaves to maximize light transmission through winter (that is, if you plan to keep your cover on year-round).