10% seems to be pretty standard but there are varied feelings about this when you talk to hunters one-on-one. I tip 10% even when the service is sub-standard because I feel obligated to. I feel better about tipping the cook when the food is really good and I'm a light eater.
I've listened to guides brag about being given expensive optics, knives and other gifts in addition to their tips. That kind of talk is inappropriate, IMO, and sounds more like suggestive talk to me.
On occasion, I feel like I've received exceptional service, or I took an exceptional animal thanks to the guides efforts. On those occasions I tip more than the "norm". I might add that I'm not always successful, but I never let that influence my decision on how much to tip.
I have talked to hunters that don't think it's appropriate to tip at all. Typically, they say something like, "I got what I already paid for. Why should I be expected to pay more"?