I normally go out broad daylight in the morning or in the evening.
If someone's out in the yard and looks, I always ask, I've never had anyone get nasty or say "no, don't take that", a few have even helped me load it up. I always thank them and go on my way.
I have a fairly regular route I stick to. You find you have certain houses that always seem to have good stuff setting out at certain times of the year.
Alot of people are happier seeing it picked up by someone who will fix it, use it, or give it away to someone who can, then to watch it being thrown in the trash truck, they'll actually set it out in front hoping someone will grab it.