I agree with the what has been stated several times. If you don't want to tip - don't.
For most of us, we've saved up some extra money for our trip, and tipping is not in the budget. No one will mind.

As for me, I like to be a 'good customer', especially if I ever plan on using their services again. When you care about the little details - fresh eggs, the best fly, finding the best 'taking' water, getting feedback on your presentation, a nice streamside lunch, and going the extra mile in general - you want them to consider you a good customer. If you are an a$$ at Denny's, you can expect a loogie (sp?) in your hamburger. If you are a bad customer on the river (tipping is definately not the only factor here, mainly it's your overall attitude), then you'll get what you paid for - probably nothing more.
I tip the lady who deep cleans our house once a month, and you can tell she appreciates by the effort she gives it every time. I tipped a guy who washed the outside of our windows and skylights because he was meticulous and didn't let a single streak go un-noticed. And I don't mind tipping a guide if I think he deserves it.

Another point - guides need to keep their base rate competivive to attract new clients. Cleary some guides are worth way more than others who charge roughly the same rates. When you find the good one, and you plan to use him again, I would advise becoming one of his 'good customers' - whether it's just being friendly and sharing some jokes, helping take down the equimpment at the end of the day, not being pi$$ed because you may have gotten skunked, or maybe you leave a small tip...