In business and in my day to day life I have always negotiated (or haggled if you prefer) when it feels appropriate. There is no point trying to get a discount from a large company when you know that you are dealing with someone that doesn't have any authority. With independent traders there is always a deal that can be done.
However, I would not usually ask a WG to discount her advertised rates. The closest I have come is when rates have been increased since my last visit, and I have asked if they want the same amount again. Based on the tone of the response I decide whether to go ahead with making the booking or not. Otherwise, it's like pissing off the waiters in a restaurant, you suddenly feel less inclined to eat there.