I collect seeds quite often and once in a while plant starts from public spaces. I do it in broad daylight sometimes during busy times and just act like I belong there, never had a problem. Private property is a little more tricky. If it's a house and I really want it I'll knock and ask, if it's in the yard proper. If it's accessible from the sidewalk I just help myself.
I currently have Japanese Maple seed from a funeral home parking lot, a really nice variety of American Holly from a drive thru bank and Southern Magnolia from multiple locations in a town near by. The magnolia is a very nice strain, lots of them in this little town are apparently volunteers from some very large ones in the yard of a church.
Nothing ruins a neighborhood like paved roads and water lines.