I guess it depends on the country.
In Costa Rica we hired a private guide. Unbenounced to us, he had purchased bottled water, coke, and beer for our daytrip, hired the best boatsman on the canal ride, and made a special request for a snack-tasting at a local beachside restaurant before picking us up. We bought him a lunch at the restaurant and were going to tip him after the tour...but he just said "All I want is for you to say you had a good time, and spread the word. That will make me happy because that is how my business thrives".
In Mexico we "bumped" into a gentleman that wanted to show us the way to the local's market for best deals. He guided us, stood by us to make sure we weren't "ripped off" and guided us back to our ship. We would have never found this place othwerwise. He wouldn't leave until we showed our gratitude, so we gave a reasonable tip and all he did was smirk and walk away.
So...I say, to your own discretion and don't fret about it too much...it is YOUR vacation, enjoy it!
Pacific Princess Sept/01 - Family in Pacific Northwest
RCI Radiance Of The Seas April/06 - 25th Annversary in Panama Canal
HA Volendam Sept/06 - Dad's Surprise in Alaska
Crown Princess April/07 - Fun In The Sun in Eastern Caribbean
RCI Brilliance Of The Seas March/08 - Here We Go Again
RCI Vision Of The Seas Dec/08 - Joining Sis On Her Redneck Honeymoon in Mexico