There is no reason to not start the fluoride as early as you can, it will no no harm, and will have those extra days to penetrate into the enamel of the teeth. I had no problems with fillings and crowns made of many different materials. The impact on the salivary glands and other vital structures is directly going to be impacted by their ability to map the treatment around these anatomical structures. Some structures will have to have radiation pass through them to reach the areas that need to receive the radiation and 100% sparing of important non malignant structures cannot be avoided. So there is always some damage to these areas. However compared to the treatments that those of us went through a decade ago, there is significant non disease involved structure sparing with IMRT.