I always use toothpaste. I love the stuff. The Master-Cleanse can really do a number on your mouth. Check your gumline, you will probably notice a great amount of tartar around the base of your teeth.
In addition to scraping all this stuff off, toothpaste improves the Death Breath that fasting people can have. Its not just MC. Almost anyone fasting goes through a period of terrible breath. The upside is that toward the end it tends to get better. Better than ever, really.
But in the meantime, I have experienced enough Master-Cleanse to feel comfortable that the benefits of brushing with toothpaste far outweigh the possible downsides I have read about.
Don't forget to brush the hell out of your tongue and drink water like crazy.