Similar industry
I work as a part time guide in another outdoor industry. I work for a large guide service as an hourly employee. I am tipped about 60% of the time. I’ve been tipped with everything from cash to small bits of gear (ex. carabiners) to Dunkin Donuts gift cards. Tips are never expected and always appreciated.
no worries
1. I’m the owner so shouldn’t deserve a tip.
2. 80% of my clients never tipped either.
3. I did that work for the fun of it. 
maybe someday we’ll just have to revisit them thar San Juan-os!
steve (typing whilst I’m looking out over the Gulf Islands in beautiful Ladysmith)
Just enough to demo a roll
I like to tip them right at the beginning, just after they say how much they love to roll. I usually tip them to the left and hard enough that they can’t just high-brace up. It gets the whole credibility thing out of the way and makes me more comfortable to have him/her tip me a bunch of times.