First let me say if this was a very special quilt that needed to be perfectly square, I would not use this method. However for daily use quilts, throws and baby quilts it is a huge time saver.
After I finish quilting, I roll the quilt back so I can see the bottom edged. With my good scissors, I just start trimming along the bottom edge. (Make sure your leaders are out of the way.)
Next, I pull the quilt over the belly bar, just a little at a time, to give me access to the sides. Trim up each side as space allows.