Collars or No?

I have three cats and they are totally indoor. The youngest is about 8 and he has not been out side since he was brought in as a little kitten found in the park, starving. The two girls, mother & daughter have not been outside since they were brought in about 15 years ago when mom was found with her kittens. We found home for all the kittens but one and kept her and mom. They are all spayed/neutered. No collars. As they do not even try to go outside they are not chipped either. They do get their shots and dental care, the male had surgery for urinary crystals a few years ago.