I've been know to start a few threads in my day, although in all fairness about half are postings of articles. In any case, I don't start as many threads on a active day than on one where not much is going on. Things have been pretty busy and my new threads have been more limited.
It is not always possible to avoid topic overlap, especially when a thread goes in one direction and you would like to followup on something made a page earlier. In that situation, starting a new thread is better than hijacking another thread. But it has to be done judiciously, or else people end up making the same statements two or three times in similar threads.
Are there any rules about starting threads? No. I have certain personal guidelines that I use:
1. No Deceptive Titles - It really irritates people when they see what appears to be announcement of a fact when it is just a "teaser" to get people to look. I can't say that none of titles have been misunderstood, but I can say that it has not been my intention.
2. Make A Coherent Point - I've been accused of giving rather long opening posts (with justice I'm afraid), but few will question that there is substance to agree or disagree with. Starting a thread that only asks a question without giving a point of view on at least why the question is worth discussing is very limiting.
3. Avoid Unnecessary Topic Overlap - We've had times when there were two or more topics with almost the exact same title. It gets hard to figure out if it is one that you've already read. The title should be descriptive enough that it is possible to understand the difference between this thread and some other similar thread.
4. Don't Start At Thread Until You've Read All The Other Threads - In some cases, the point has already been made and which should be acknowledged. It also helps in establishing what is different from the other points. It is also helpful in catching where an article has been reprinted, although I admit I've missed a few cases where links were given but not the entire article.
As I said, these are my principles. You can use them or not as you wish.