Alot will depend on your level of fly fishing experience.
What do you want out of the guide? Casting lessons, reading water, fly selection, new techniques etc? Catching a ton of fish? An enjoyable day on the river? Floating and wading?
Tips are not always required but if the guide was receptive of all of your needs and performed them well, then yes a tip should be handed out. I think somewhere around 10-20 percent of the trip price would be acceptable.
Be aware that we are no longer in your grandfathers era and that most guide today because of liabilities do not allow alcoholic beverages on a trip. They may have an adult beverage after the trip is over.
Call the guide in question and ask him what is expected from you also. Talk to him about your experience. Alot of guides will provide you with the proper equipment if you dont have any, but you do have to ask.
I personally dont like a guide to do everything for me. I can tie on my own flies etc. I want the guide to show me the intricacies of the river or stream.
Read this:
http://www.ehow.com/how_13222_hire-fly-fishing.html
http://www.bozemanangler.com/guide.html