You're definitely going to want to work on your accuracy. From the technical player's or perfectionist's standpoint this is a must, but it also just makes things better for you. Improving your accuracy here will help you on other chords and even when picking single notes; it will probably also positively impact your timing and help you to play faster or for longer due to less motion and obstruction.
While not unique, my method of chording power chords is a bit uncommon. If I were to play G5 for example I'd have the tip/end of my index finger on the 3rd fret of the E string, the tip of my ring finger on the 2nd fret of the A, and the tip of my pinky on the 2nd fret of the D, and my fingers arched. Quite different from the barre or partial barre many use with the fingers laying flat, but consistent with how you'd play many chords like a standard G (320033).
This makes it nearly impossible for me to mute the other strings, and I don't try to anyways, meaning my accuracy has to be "perfect". "Perfect" in quotes because there's really still quite a bit of leeway with the gaps between the strings and so on. It's easiest to play power chords based on the E string, and I tend to prefer the sound of the lower strings, so I'll often play 799xxx rather than x244xx for a B5 and so on.
There are three basic methods of avoiding hitting the other strings while strumming:
- Halting the strum after hitting the strings you wanted
- Strumming on an angle to avoid the other strings
- Strumming on a curve to "swoop in", hit the strings you want, and "swoop out"
(1) is pretty obvious, but I felt like making a picture anyways:
For the 799xxx example you'd just hit the E, A, and D strings before cutting the strum short. This is hard to get right initially and it can be tiring since you're adding in the effort of stopping your momentum so suddenly. It's also quite difficult to play fast. I consider this sort of a hack, although in certain cases it's useful. It allows you to give all 3 notes in the chord essentially equal power with a bit of a harsher sound, and you can create a nice effect by adding in a palm mute when you stop your momentum (essentially smashing into the strings). Finally it's pretty difficult to play chords that aren't based on the E string; to strum straight without hitting the E you need to rest your pick between the E and the A strings before the strum, which doesn't give you much room to accelerate and so on.
(2) is my preferred method when playing chords based on the E string. Basically you're hitting the strings like so:
This is pretty easy to do; once you get the angle right, you just repeat ad nauseam. It allows to to make powerful strokes with as much of a windup as you want (windmilling and so on) if you want to, as well. It allows you to put more emphasis on the bass note of the chord since you're hitting it more solidly with the pick than the other strings. Obviously, it won't work for a chord with a muted E string and so on.
(3) is like this:
This is the most difficult and also the best method. By altering the shape of the curve you can emphasize any of the three notes in the chord and it allows you to play chords based on any string easily and consistently. To play this right you need to get really familiar with your instrument — the size of the gap between strings, how much a particular pick rebounds when you hit a particular string at a particular angle with a particular force, etc. — and you can only do that via practice, practice, practice.
It's easiest to use this method by strumming largely with the wrist, which allows for soft chords much more easily than the other methods. Harder strumming can get tiring and even painful if you rely only on your wrist, though, so you'll have to learn how to twist your wrist just enough and with the right timing that you "swoop" in and out hitting only the right strings during the strum. Again, this takes practice. It really shouldn't take you too long, but you do need to purposefully focus on it in order to get it right.