It's isn't good or right to only believe something. You should know where that belief comes from. Especially when it has to do with physical consequences. (Like drowning kittens. You don't have to drown them 9 times to make sure they're dead.)
For example, I can believe that quicksilver is the elixir of life and make a thread and say. "This is only for alchemists who believe that quicksilver is the real actual elixir of life. You don't have to believe it too, but don't post if you have any concern for our health and safety." No. They should post that they disagree, because quicksilver is a deadly poison. That is not a matter of belief or opinion. It's a fact that quicksilver is not the elixir of life. It's a fact that cats die when they're injured or ill, just like we do.
Cats are impressively agile, and magicians throughout history have loved prime numbers like 7 and 3. And 9 is three 3's. So, it was just a way to express how impressed we are with cats' agility. That's where I believe this belief comes from, too.