I used to trim mine, then sew on the binding with a 1/4" seam. However, now I square up the quilt first and mark the squared-up line with a Sharpie permanent marker. Usually I do a fairly big zigzag stitch outside the marked line to stabilize the edge, then rough-cut outside the zigzagging. The marked line clearly shows the edge of the quilt, so I line up the cut edges of the binding with the Sharpie line and sew the binding on.
Only after the binding is sewn on do I cut the excess batting and fabric away. Before I cut, I pull the binding over to make sure the binding will be filled. This allows me to make any minor adjustment in the 1/4" seam allowance so that the binding folds over the edge perfectly.
This system works really well for me.