When I had a 4-wheeler, I always strapped it down so that the straps held it from going either forward or backward. That way there wasn't the danger of cracking the front of the bed. The bed is not steel, so you shouldn't strap it tightly against the front of the bed.
Since I added a winch on the front of my Wolverine the push bar on the front of the ATV sits right at the top of the bed but the tires still hit the front of the bed before the push bar contacts the glass. Every ATV will be a little different. Just be careful loading it, especially the first time.