@Eddy Dumire
I agree with others to check out your insurance first.
We also have a pet addendum that requires both cats & dogs to be spayed / neutered and that the tenant agrees there will be no breeding of animals in the units. Animals have to have updated registration as well and have their vaccinations.
Our pet policy says we have to meet the animal. Regardless of breed, the dog should be calm and friendly. They may be overwhelmed and a little ADD, which is fine, but when you put out your hand and say their name, they should acknowledge you and sniff in your direction even if they're too ADD to come to you.
A dog who immediately starts barking uncontrollably, or desperately pulling at the leash, or running off ignoring both you and the owner is a dog that still needs training and maybe isn't what you want in your home.
I wouldn't decline a german shepherd that was calm and older, but might decline a young one if I felt the owner didn't have a good handle of it. Young dogs can be brutal on a rental. They've been known to eat the floors, eat the walls, chew the doors, bust through doors, tear out screen doors / windows, dig up your garden...