It shouldn't hurt the bead (the part that seals against the rim) but it could easily damage the sidewalls. As Bird says, enough rear counterweight will keep the front end from being lifted off the ground. And another thing it accomplishes is that it reduces the weight on the front tires, as long as it's enough to keep the rear tires on the ground, of course. (If you need 500 lbs on the rear to keep the rear tires on the ground, and you put 400 back there, when the rear tires come off the ground, all you did was put an additional 400 on the front tires.) Think of the tractor as a being a fulcrum with the pivot point being the axle with the most weight on it. You could conceivably put enough weight on the rear that the front tires would come off the ground. If you did that, there would be no weight on the front tires until you put a load in the bucket, at which point the weight on the front tires would be the amount needed to bring the front end back down plus the balance in the bucket.
