Actually, the manual states use only the bottom holes for a rear blade, but many people were reporting top link damages when using a boxblade or rear blade in the top hole. I don't pretend to understand how that little bit of spacing could make much difference, but I mostly only use the bottom two holes.
I had this discussion so long ago, I don't remember all the particulars, but it was all about angles and stresses.
The other advantage to using the bottom hole is that you get a bit more lift which is handy when boxblading or rearblading.
Charlie, as long as you're getting the amount of lift you need, you can use the top hole. If you experiment by moving it down, you'll see that you get a little bit higher lift.
Damage is not really all that likely unless you really grab hold of something that brings you to a slamming halt with the top link in the uppermost hole. Even then, the damage would probably only be a bent top link (prolly while pushing back) and not the mount.
John
Edit: Most implements can be run in any of the holes as long as you're getting the lift you need. Mowers in the top hole are okay if it's getting high enough for you to cut and lift right. Post hole digger would probably clear better in the bottom hole, as you get a bit more lift.
Basically, not get anxious about damage, as I said, at the most you'd prolly only see a bent top link if you hit just right.
J