Should I declare a minor if it is not required?
Why not? What's the disadvantage? An afternoon of paperwork? One extra class you have to take?

Compare that to the advantages: Let's say you are looking for work, can't find any in your chosen field (medicine), and so begin looking to work in a chem lab. Being a "medical lab science" major doesn't necessarily mean you know how to do a lot of chemistry, it may just mean you know things about handling biohazards like blood carefully. A hiring manager who doesn't know your school or that program doesn't know how much or little chem is required for such a major. You can't prove you know any chem unless they ask for a transcript or you have relevant work experience. But having "Minor in Chemistry" on your résumé would show that. So you could potentially land a low-level chem lab job easier that way.