I've noticed, and mentioned, that the tipping guidelines haven't changed since I started traveling with GCT. And that was in 1999! So if I'm really pleased with the Program Director, I tend to give more than $6. That's usually the case. In fact, I can remember only one or two that I could consider only satisfactory. My sister had a different experience a number of years ago on a transCanada trip that is no longer on the itinerary. That Program Director never waited until the entire group was off the bus before he started talking, refused to answer most questions, and made remarks about certain of the group that most people would consider anti-semitic. She did report this in her evaluation.
As far as the local guides and bus drivers are concerned, I tip on the high side of the guideline and usually round up. We discovered that the guides on the shore excursions from the river ships that if the guides feel one of them has been stiffed, they will combine their tips. There was an instance when the radios had first been introduced (2006?) that one of the local guides refused to use it, and the people in her group feeling that they had missed a lot because of her intransigence all decided not to tip her. The other guides made it obvious that they were making it up to her. Commendable on their parts but, of course, they have to work with her.
If you're following the OAT forum, you'll see a lot of discussion about tips recently.