There's no rule of thumb. I've decided against it. Why bother? Rules are just an easy way to get back at someone by twisting what they've said or done and shoving it up their nose.
I do not believe in an answer. I believe in its plurality. I believe in secrets. I believe that some truths will never come out, and that it is better that way. I believe that we do not know the same things, but that doesn't make any of us smarter than the other. I believe all that. Today.