Originally Posted by
morgothaod
Is it a good rule for saddle height that my heel with the pedal pointing down in line with the seat post should have my leg completely straight?
No, but it's a good way to find a place to start with your seat height. Though if by completely straight you mean you are using all your effort to make your leg artificially straight, probably not.
Once you've got that place to start. Try it out and adjust as needed from there.
My objection to what you said was the term "rule". I think that makes too many think this is what it must be and thou shalt not deviate one iota.