Re: How much does being in tune/key matter?
Just make sure if you do go out of key, it's for a good reason, example driving towards your tonic, also augmented chords are find too, just do what sounds good to your ears! Build some tension by taking your bottom note, and playing with an extra note on the chord or something.
Also, you can just take the root, or whatever note and move it an octave higher. 7th seems cool on the dominant, adds tension, no one expects you keep going, then resolve the progression.