I'm not a guide - but I've hired guides about 5 times (not counting 2 charter fishing trips).

Worst trip - Jet sled 1 mile down the Sky, anchor the boat, drop some plugs, and sit and BS all day trying to keep warm in front of the propane heater. Occassionally jumping up (if it's your turn) to crank in a chum on that 25lb cable. Woohoo. I think we moved about 20ft all day. My dad had fun, and 2 out of 3 clients got their 2 chum limmit - but I would never do that again.
I don't think I tipped - but it was my first trip, and I didn't really think about it (plus I was paying my dad's way as well).

Best trips - fly fishing for salmon/steelhead on the NF Stilly, Sky, and an OP zipperlip. We floated in personal pontoon boats (awesome way to go), and learned a heck of a lot about reading the water, fishing under differnt conditions, choosing the right equimpment, and tons more. 2 trips I ended up with sore arms from all the fish, 2 trips we got skunked. He worked hard each time, shared 'secret' guide tricks/locations/flies etc, made us laugh, and taught us a lot. He got a tip every time.

If you can afford it, and he wasn't a complete looser - a little tip seems pretty reasonable to me.