Unless you plan on spraying thousands and thousands of tall beans for aphids or fungicide I wouldn't waste my money on skinny tires for the tractor unless you need new tires anyway.  We ran 18.4s and on beans knee high or less they didn't do much damage and dropping to a 14.9" tire isn't going to make much of a difference.  If your that worried about crop damage in tall beans your probably better off hiring an airplane to spray them anyway.