I would always put the title. Generally people don't use the post-nominal KBE or DBE on a letter, however it is the most formal way of addressing the envelope.

By the way, the convention is "Sir Elton John" or "Sir Elton" but never "Sir John"

Those living in America would still use the title, though watch out for those (such as Bill Gates) who are KBE/DBE but as they are not part of the Commonwealth cannot use "Sir" or "Dame". Bill could use William Gates III, KBE but not Sir William Gates III, KBE.

I was wondering your thoughts on writing someone who is Knight or Dame in either a British or Commonwealth Order.

Lets say you are writing to Elton John do you personally address the envelope as "Sir Elton John, CBE" or do you simply put "Elton John"

How about when writing someone like Elizabeth Taylor who lives in the United States? Do you still address her as "Dame Elizabeth Taylor, DBE"

What is your views on this? I mean on the envelope by the way.