It is not a hardfast rule at all.

Typically, the caveat, "with taste" should be followed.

Which to me means, if it works, then you can do it.

And if it doesn't work, then by all means don't do it again.

Then there is the beginning Hammond Organ player who tries to make *every* dominant chord a full 13th stack, close harmony...

That is bad taste at its finest.

But give a hard listen to the man who plays rhythm guitar behind BB sometime - AND his keyboardist.

Some complex feces goin' on there.

--Mac