I don't think there can be a set guideline as to when to tip and when to not. It is a personal decision and varies with the circumstance. In general, I think that tipping has gotten way out of hand in our society with too many people having their hand out. Still, I try to be fair when deciding whether or not to tip an individual. For a shore excursion, I generally don't tip unless someone has gone above and beyond to make the trip enjoyable. For example, a tour guide who goes beyond just narrating the facts and really makes the trip FUN or an employee who in some way lends a hand to me personally.
Basically, I like "Mel's" suggestion of tipping when you feel that the excursion was more than you expected (which for me, rules out tipping on most ship-sponsored tours since they are so overpriced anyway). Why would I give more money if the trips turns out to be exactly (or less than) what I expected it to be for the price I already paid?