I've mounted fatter than recommended tires on rims and had no problems...the issue is really that really wide tires on a narrow rim can exert a lot of force if inflated to high pressure which can damage the rim or blow the tire out. Of course you don't need much pressure in a high volume tire so unless you make a mistake filling the tire with air this isn't much of a concern. You can also have issues if you corner hard with very low pressure, but again that's not going to happen unless you really under inflate your tires.