The more informed the readers are the better, but anyone will do as a means of checking general clarity, style and sanity. if your reader doesn t understand rewrite the proposal so they can t not understand. If they can t see the value of the proposed research, rewrite it so they can. A good idea is to ask them to spend 5 minutes on it, then give a view. This is the amount of time a reviewer will often give the proposal and your proposal needs to make an impact quickly. Further things to bear in mind are that some parts of your proposal are potentially more important than others.
First impressions count.
A reviewer will often start with the summary bits of an application to get a feel for what you are doing before ploughing through the detail. A common view is that if you impress the reviewer in your first two sentences the good impression tends to stay. Make them snappy, clear and informative. All sorts of reasons, including bad luck - although its amazing how lucky the people are who work hard at grant proposals. It is a good idea to treat these issues in the same way that you treat those issues explicitly stated in background information.