Trespassing is only when you are on property without business there and the property is posted as such and is fenced (those with acreage that deal with this issue feel free to correct me). As a mailman, delivery driver, meter reader, etc., you are not trespassing. Additionally, the owner of an animal is responsible for the control of that animal at all times. If the property owner puts up a 'Beware of Dog' sign to warn me I respect that sign and will not enter their property. I will try to find an place I can safely leave the pkg, otherwise I will have the clerk postcard it. I also have notes in the diad indentifying addresses with bad dogs.
In the county I deliver in (Marin in N. Cal.) you can be fined up to $5000 for NOT reporting a dog bite to the authorities.