Since I started this thread, I'd like to chime in one more time. The hunt that I am going on in SA (in 3 weeks!) was won at an SCI auction/fund-raiser dinner about six months ago. I got the hunt for less than half price. The PH who donated the hunt was also present at the dinner, so when I won it we had time to talk and discuss some hunt details (but tipping never came up). Yet, I am comfortable that I will be able to provide this chap with a generous tip (if he does a good job) and still feel like I had a great bargain.
Next year when I go to bid on another hunt, I think I'll look for something in Australia or New Zealand - that way I'll get a great deal on my hunt, plus save another pile of cash by leaving my tip at home - regardless of how well my guide prefroms!