I don't have a crupper, and my saddle doesn't shift forward going down steep hills. However, that probably has to do with the fact that I get off and walk down anything that could be considered "steep". ;)
I don't think a crupper would be a great investment for me at the moment because I do get off and walk so I don't have saddle slip issues - yet. I'm sure if I need to ride down a hill, I'll be missing one.
On a related note, I make my own rope and tack and have a design for a crupper in each. However, I'm concerned about rubbing. I'll try to get a picture of what I plan to do so you can get a better idea, but what should I pay close attention to and where is the horse most likely to be uncomfortable or get a rub?