I actually have never been turned down when asking and I have gone into some pretty sketchy locals for permission.... but I have done a ninja move or two before too .... mainly on university or hospital grounds they always have the nicest trees... but yes, ask first you would be surprised how many more trees that gets you access to too. There is almost always another good tree they know about. That reminds me I need to stop out at the local hospital - there is a ornamental I need to snip from and a weeping crab on campus.