In cases of many older celebrities, home addresses were generally the only way of contact we had.

I think over time, newer collectors took to not wanting to use home address, as if they're trying to protect the celebrity like a friend.

I doubt very many who are thoughtful enough to sign care whether you're sending to home address or not.

Bad idea. Most will send you your letter back or not reply at all.

I think it can be justified in some cases, especially when the celeb is no longer active. Then a) the volume of fan mail can be expected to be quite low, and b) there may not be other options such as studio. I have a good example - there is a singer/songwriter who had a multiplatinum album in 1990, and haven´t released any material since then. He doesn´t have management or a record company, and plays only a few small scale concerts per year - 2013 he made exactly 1 (!) live appearance, this year he played a total of six shows. And he isn´t really accessible for autographs before or after shows. So the options is through the agency that books the shows, and I am not sure how much contact they have with him at all, or the home address which is publicly available.

When I started this hobby I decided that I would never used the real address of a celeb, something like a personal ethical code, what about you?