#1
I listen to and play a lot of metal music, so power chords come up a lot. The only problem is, they often appear in the root-fifth-root format. I've always played them holding the lower fret with my first finger and the higher fret on both strings with my third, and all three notes sound just fine. The thing is, whenever I look at guides, live videos etc, it always shows the use of the fourth finger to fret the highest note. Is there a reason for this, and if so should I train myself to play power chords using three fingers instead of the two I do use?
#3
I'd say that if you're comfortable doing power chords one way, don't change it just because a guide says it's done another way. I personally fret my power chords with my index finger on the root and my pinky finger flattened on the 5th and octave. It just depends on what's comfortable for you.
#4
That is a quite common question. There is nothing bad in the way you are playing power chords, but if we strictly look at the technique as a way of achieving best results with minimal effort, playing the highest note of the power chord with the fourth finger would be better because you apply less pressure on the strings and your hand is less strained then by barring the two strings with your ring finger. Because of that, you should be able to change chords more quickly and play over a longer time period without getting fatigued. But, if you are a more experienced player, your fingers should be strong enough to play in both ways without feeling the negative consequences. I have noticed that I made less string noise when changing chords the same way you do, but later I switched to holding only the root and the fifth, without the octave, because that makes the power chord sound more defined and a bit darker.
#5
Know both ways, but either is generally fine.

Using 3 fingers probably makes it easier to slide between power chords with and will let you use your middle finger to add extensions like the major third if your power chord is on the 6th string. Using just the 3rd finger also has advantages though, for example if you're playing a power chord on the 5th string, you could turn it into a sus4 chord by grabbing the 4th on the B string with your free pinky.
#6
I think it is handy to know both ways. In some songs where I play a lot of power chords, my hand starts to cramp. When that happens I switch to playing them the other way, and throwing in some open when I can.
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#7
People usually use the index, ring and pinky finger for those power chords because it's far easier to change to another chord shape that way. If you need to play a b5, that's easy: just move your ring finger down a fret. If you need to play a major chord, put the middle finger down... et cetera. If you barred the fifth and octave, you would need to move your fingers more than necessary for different shapes. It's all about convenience and efficiency.
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#8
I do both. It's a little easier to do riffs using your pinky on the root octave. For me at least.
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#9
If you play both of the top music notes with one finger, that finger has to flatten.

If you use one finger on each string, then the tips of the fingers can stay curled.

I sometimes use the first style, but usually use the second.