I use 2 1/4" also, except on miniatures where I do 2 or 1 3/4". If you do wider binding you will have a larger width at the back than the front, like a frame for the back. Some people like that and it does look good--the front will still be the narrow width unless you have taken a greater than 1/4" seam while sewing the binding on. I don't think there is a set rule however I think 2 1/4" is the most popular.