Just for the record, I don't think it's crazy; I think it's awesome. I do the same thing. I'm also a server, and I can tell you that (if I wasn't a coin collector) I wouldn't care if you got the change from under my table. Someone who's tipping change doesn't need to be going out to eat, and if that was her tip, she'd much rather you pick it up and enjoy it than suffer the insult of, not only getting change for a tip, but also having to pick it up off the floor.
Keep spying the change. Your wife will be thanking you the next time you find a coin worth a couple of hundred dollars.