The middle of my rooms are. We're not talking about listening position for monitoring here. We're talking about minimizing ac gtr untreated-room boominess. And, even thinking for a moment about nodes, where's the mic going to be if the guitarist is in the middle of a (usually) fairly small room? Not in the middle.
Have you used hypercardioids on guitar? The rear lobe on the several I have is a very subtle thing, and WAY offset by the very full null at 130 degrees.