Pinkiris: Some quiltrs find that sewing off at an angle into the corner helps in making a good mitre. The normal procedure is to sew the binding (approaching the corner) to the 1/4" point from the end. However, this is not always true if your seam allowance is any width other than 1/4". I believe the assumption is that all binding seams are 1/4". My bindings are 3/8" and I have even made them wider if I want a different look. So lets say - sew the binding to the point that represents the width of your seam. I usually stop one or two stitches before I get to this point and backstitch a couple stitches. Not sewing to the exact point of your seam allowance will leave a bit os space to accommodate the folds in the binding. If you angle to the point you will need to sew to the exact spot the width of your seam allowance, otherwise the corner will not be a 45 degree angle.
One of the comments above asked if she should reduce the width of the binding to have a good fold. I have found that the info that is missing in most tutorials or instructions is that the width of the cut binding has a direct relationship to the width of your seam allowance. Usualy a 3/8" seam allowance requires a 2.5" wide cut binding.
The 1/4" seam allowance will require either a 2" or 2.25" cut binding.
It is good to test after a few inches of sewing to make sure the seam allowance is good and the binding will fold to the back with the batting and quilt filling the binding. If the fill is not good then the seam allowsnce must be adjusted or the binding recut.