it is NOT absolutely required to trim away the excess behind the sky. it is advisable - especially if you plan to hand quilt - because it leaves less bulk when you start sewing your patches and blocks together. however, if you've gone so far along and trimming away would be difficult, do not ... i REPEAT ... DO NOT feel badly. Many quilters deliberately leave it and don't trim at all. (if I remember correctly, Maryellen Hopkins - who gets piles of money to travel the country teaching - doesn't trim the hidden excess.)