When I play power chords on the three lowest strings, I'm fine. But when I play power chords on the A, D, and G strings, I mute the E with my middle finger. Am I doing this wrong? I read something about using your thumb or the tip of your 1st finger to mute it.
Which is correct?
any of the methods you mentioned will work. What you do is fine, there isn't really a correct technique. Whatever works, works.
"There he goes. One of God's own prototypes. Some kind of high powered mutant never even considered for mass production. Too weird to live, and too rare to die."-Duke
When I'm playing octaves of notes on the 5th string, I will use my 2nd finger (middle) to mute the low E string, if this is what you are talking about, then I do the same. There isnt necessarily a "correct" way to do it, its just personal preference and what feels the best to you.
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I've never even thought about muting the E string, I just lay my fingers down to mute the e B and G strings, I guess you could just practice not muting the E string and it could work on your accuracy with strumming.
the thing is that some people like me go crazy when we strum so we rely on muting
I just use a barre chord (where the power chord comes from).

It's easier if you don't have good accuracy...although somesongs you have to use power chords there's just no way around it, but anyways...i don't know if i helped or not, but whatever.