In the land of "Live Free or Die" (NH motto) it is a courtesy that you "should" ask for permission of the land owner to cross his property.
It is only tresspassing by law, if it's posted every 50' around the whole perimeter of your land AND must be signed and dated EVERY year.
Stupid law, who has time to do something like this???
Gotta love the laws, as they protect the guilty!