I look at the fabric and I will only miter if the design in the fabric will look better. I think it's harder to get a flat border when you miter it so I always try to be real careful in measuring.
One of the easiest way to miter is to sew on all the borders stopping 1/4" from the corner and locking the stitch. Then play around with the corner folding the fabric to where you want it then ironing it down and I like to put a thin strip of double sided fusible there to hold it in place while I sew it. Some people will top stitch it down with invisible thread.