I don't have much first-hand experience with multiple-stop routing. Most of the time I just need to enter only one single destination. But if I recall correctly iGuidance should be able to remember the destinations you entered into the itinerary, without you having to click any "Save" button.
However, after you plan the next trip, the previous destinations are gone. If you wish to look for a work-around, try to look into C:\iNav iGuidance. There should be a file by the name of itinerary.dat. Copy and paste that file to a different location for later use, as it contains the stops for your curent trip.
I haven't myself done it, so I don't know for sure if it works. But I think it could work. Give it a try, and let us know.