I always use my seatbelt, even on flat land while just mowing our tabletop flat yard. If, for some freak reason, you'd tip, or get jolted hard, like hitting a frozen ice chunk or hidden rock with the bucket, you're gonna stay in the seat and under the ROPS and canopy. Just remember to hang onto the steering wheel and don't instinctively reach with your arm to break the fall.
I've been up on two front wheels hundreds of times on the 425. That doesn't bother me in the least. I've been up on two side wheels twice, and it scared the bajeezers out of me. Once, I thought it would be interesting to see how fast I could mow and how hard I could turn while maintaining that speed. I got up to full speed, 8mph, and cranked the wheel to one side a bit and kept repeating with further and further cranks. Things were going well. It was increasingly uncomfortable the tighter I turned at 8mph. Finally, on one try, the right tires came off the ground and I thought I was going over. I popped the mower out of float as I cranked the wheel back and it came down. But that was enough. It can be tipped over on flat ground if you go fast enough and turn hard enough. Had I not had the 60" mower on the FEL arms, I'm fairly certain I could have rolled it onto it's side with little effort.
The other time I had it on two wheels I was plowing snow and went out in the street and started doing donuts. Of course, you can't do real donuts because you can't break the rear end free because of the 4 wheel motors. But you can make stupid tight turns. Well, I caught some pavement on the outside tires and the machine lurched to the left and the right tires hopped off the ground. Had I kept on the forward pedal, I'm pretty sure I could have driven it over onto it's side. But I let off and down it came.
I know these things were intentional, however, one could easily get into a situation unintended and the seatbelt could keep you less injured. Up to you, of course. I just make it a habit to click it every time I get on so that I don't have to think about it. I put my pants on in the morning and put my wallet, pocket knife, fit bit and cell phone in my pockets. I put my seat belts on in the car and truck and tractor. Good habits. :laughing: