How wide your ideal seam allowance would be depends on how wide you cut your binding strips and how thick your batting is. As a rule-of-thumb, when I cut 2.5" strips I sew a 3/8" seam on the binding. My typical batting is Hobbs 80/20. If you are using a very thin batting, you might want a larger seam allowance; if using a very thick batting, a narrower seam allowance.
Be aware that the size of the seam allowance affects the look of the finished binding. I would not sew more than a 3/8" wide seam allowance because anything wider than that makes the binding too wide for my taste. For show quilts, narrower bindings are preferred, which typically requires cutting 2-1/4" strips and using a 1/4" seam allowance.