Make sure it's the one in the same key. If you wanna get jiggy, play the one that's a diminished fifth away from the key.............

...needs more explanation for us 'great unwashed'!..

Mo

So your pointer finger is on the root, say C. Take your middle finger and put it one string over and one fret up which is F#. Voila, diminished fifth! And not a fifth of sake!

Would not one finger over, one fret up ( from C )correspond to G#, the flattend 6th?

Mo

Let's see, put your finger (pointer) on the C, 3rd fret. Now put the same finger on the F, 3rd fret. Now put the middle finger on the F#, 4th fret. That's what I was trying to describe. Whew!

Rules??? Never!!!! However, what sound are you going for? As with blues music, both the minor and major 3rd are used while hipper, "outside" sounding pentatonic use can be heard in the post-bop jazz world. Try playing a D, E or an A minor pentatonic over a Dm7 and listen to the difference.

As for the power chord issue, listen to the overal tonality of the phrase because there might be an instrument, including the vocal, that is supplying the third. I was subbing on a classic/hard rock gig (which had charts?!?!?) and somebody called out Warpigs by Black Sabbath! This has lots of power chords and a minor tonality but the vocal has a major 3rd in there that could fight with the minor pentatonic ...in theory. But, in practice, it sounds great and adds to the dissonance (clashing of sounds) that makes for a great rock tune. Your ears always know best.