Go for the minor if that one extra class is a class you think you will enjoy taking or are interested in. If there is a different class or academic extra curricular activity (research, internship, etc.) that you'd rather do, you should do that instead.
Double majors and minors are usually not that meaningful to employers and graduate schools as most people think, unless they are in a very unrelated field. You can always make your resume highlight the classes you took in the field most applicable for their job. Very few companies or graduate schools will care that you have a minor, especially in a field as related to medical science as chemistry. Even if taking this chemistry class prevents you from taking a different chemistry class you'd prefer to take, take the more interesting one, and then highlight the things you learned on resumes and interviews. If you'd take the class required for you minor (after reading the course description) even if this minor wasn't offered, definitely do it. However, I suppose you wouldn't have asked the question if this is true.
Personally, I really regret taking the one extra class I did to complete my minor. It blocked me from taking a really interesting project based class in the same field. People came out of that project class with great experiences an exciting line on their resume like "built ...." or "Developed a new theory of ..."
As for when to declare the minor, I really see no advantage or disadvantage for doing it later vs. now. You might as well declare it now, so you don't forget, I guess. Usually no university privileges (like priority registration) are associated with having a declared minor.