Your mouth will most likely undergo changes during chemo. I got dry mouth during some cycles of Taxotere. I was lucky not to have developed much in the way of mouthsores, but many people do get them.
I think the key thing is to try to keep your mouth clean. I got a very soft bristled brush and brushed a few extra times a day. I also used Biotene mouthwash, which is alcohol-free, and helps with the dry mouth.
I did not have any problems with cavities developing, and I have lousy teeth, but I do know that many people do get cavities during chemo.
If you develop mouthsores, there are several mouthwashes that your oncologist can prescribe to help with the problem.