The golden rule is to work with people who value your work as you do. Sure, there may be times when you make a choice to work for less than full fee. Do it for love of the work, or as a contribution, or simply because it's something you want to do. Never do it because a client is pressuring you.
Nuff said. Discounting is a discussion that shows up persistently AND consistently. This little video should go into your bag of tricks when it comes to dealing with the topic!