I think adding minerals is always a good idea. Whatever their body doesn't need or use they will pee out so there is no harm done. My horses always got supplements. Not my horse is out in a pasture with other horses and I board so getting a new black is not an appealing thing to me but if I had my own place I would get one :)
This is not true - Horses will not "pee out" the minerals. Horses can get too much selenium which is about as bad as not having enough selenium.
You need to contact your vet and ask if your area is deficient in certain minerals. In Michigan, selenium deficiency is a big problem, but not necessarily in your area. And, you have to be careful not to give them too much or it can be toxic.
I would suggest asking your vet - they should know what problems you have in your specific location. And, they can suggest what you should do.