Personally, as someone who certainly tips in restaurants and some other situations but doesn't go overboard--even though I travel a lot, I'm never sure what to do in so many of these gray areas given that I don't want to universally just my wallet.

For example, I take a limo service that's pretty cut and dried. Pick me up, drop me off. There's a tip field when I sign the receipt. I don't add anything and I've never gotten a dirty look or a suggestion that I might have "missed something." I assume I'm not being horribly cheap.