sure it's fun to dig, but all of us are out to find cool stuff to keep. If the dig is what it's all about for you, I assume you are running your machine wide open and digging all the nails and foil too?
I'm a pirate. I keep what I find or I don't hunt. I love returning lost items, and will give up a cool find if the spot was good as a thank you, but I'm out looking for goodies to add to my treasure chest.
Think it's pretty obvious to the homeowner, that you aren't asking, because you like digging up peoples yards. Metal Detecting, is treasure hunting. If a homeowner is generous enough to let you hunt, they are more than likely going to let you keep what you find. There are a lot of greedy people in this world, and you've already met a few, and will meet up with a bunch more. If I was finding good stuff, I'd want to leave on good terms with the owner, so I could return, and hunt some more. Chances are, the neighbor's homes will be good as well. If you do get a greedy homeowner, you can just leave him with his fist full of happiness, and go on to the next house. What are the chances of two unreasonable, greedy individuals living next to each other? The only thing slightly deceptive I do, is not clean anything while on the property, unless the owner really wants to see, or is interested enough to want some of the loot. Just a bunch of dirty junk. If the owner wants some, I just want to get a few pictures. Must people aren't really into dirty, rusty metal, and aren't going to dig around in my finds bucket.
A happy homeowner will let you come back to hunt some more, if you want. A happy homeowner will answer the neighbor's questions, about what you were doing, in a positive way. Wouldn't hurt to leave some business cards for the curious. A few bucks worth of silver would be nice, but it's the adventure and discovery that makes it a fun hobby. You can make good money buying a selling precious metal, coins, without digging.
Yeah, sometimes, you might end up doing a little charity work, but it's something fun, something you enjoy, it's not going to kill you, and it will often open other doors and opportunities.