How do you decide if you really have a problem? I certainly have some "laxness" after several childbirths, but my doctor hasn't indicated there is a problem

It depends. It depends on what your symptoms are. It sounds like some concerns about changes in your vagina from the question. If your physician has not commented on anything, then "structurally" things are likely normal. But if you have a concern about how your body is working now (urination, sexual performance, etc) you should directly ask your physician. These topics are easy to ignore if not mentioned.
Does it bothers you? So if it isn't bothering you, than what the problem? If you want a cosmetic correction and tighten up your "laxness" - than it would be a different approach.