I am using Marty Schwartz's video guide to learn Back in Black. I've had my guitar for a while but never really progressed more than a beginner and I want to stick to it because I know it'll be fun when I get the hang of it.

Here's the video I'm using:

I see that there are multiple different ways to play notes. I like to follow videos as they teach you good habbits, and although it might be slightly harder to play a song, this sets you up for the long term

For example, with the first chord he plays, his open E powerchord (at 1:20 in the video) and he uses his index finger, his middle finger, and his ring finger (to mute the g) in order to play the E powerchord.

However, you can also play this powerchord using just one finger, your index finger, to press down on the A and D and to mute the G.

Later in the video (at 2:30) he uses one finger to play the A powerchord. However, he does not use one finger for the previous E powerchord.

Whilst I am using this video as an example, I am more trying to develop a general rule for myself on to how accurately I must follow guitar training videos. 

I know that you should definitely follow them in terms of playing songs like Smoke on the Water with powerchords instead of playing them with one finger (or it will be harder to learn powerchords as your brain remembers how to play it using single finger instead of using a powerchord).

So overall, how accurately should I follow guitar training videos if I want to not learn many bad habbits? And in which cases is it fine to deviate? (For example, maybe it's fine to deviate if I am using one finger to cover 3 strings instead of using 3 different fingers to do so, however, it is not recommended to deviate from the videos in terms of playing a song using single chords instead of powerchords).
i'm not sure there's really an easy answer to this- some things you'd be advised to follow exactly, others are more preference and opinion. the longer you stick with playing the better you'll get at figuring out which is which.

it's also worth bearing in mind that some beginner lessons will tell you to play things in ways that more advanced players would ignore. for example, that power chord thing you said- i'd say barring with one finger is arguably the more advanced way to do it.
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I've found that the video lessons vary widely -- it sorta depends on whether you want the gist of the song or an accurate rendition of a specific recording.

One of the best (if you're playing classic rock) that I've run into is the "ShutUpAndPlay" series, which is available on both YouTube and on his own website ( https://www.shutupandplay.ca/ ). He's especially good at getting accurate chord inversions and the little details in rhythm and leads that make a difference. He'll also have information on second guitar differences in chord patterns, etc. A few of his videos (primarily Eagles things) have been silenced on YouTube as copyright infringements, but you can actually buy the videos (sound intact) and download them from his website.

You'll get a feel for what's accurate and what's not after a while. Playing a simplified version of a song won't get you very far in the long run, and correct chord inversions can make a big difference in getting closer to the recorded version.

I've noticed this is equally true in keyboard video lessons -- some are bang on, some are more interpretations or beginner approximations.

I have a story about transcriptions (accurate music done from the recordings). I've had a guy who has an amazing ear do really accurate transcriptions from recordings, and in one case I was going through transcriptions he'd done of a Dave Brubeck album when I came to one chord that was simply unplayable. I called him on it, and he said, "Nope, that's exactly what he played." I went back and looked. I have really large hands, but this was a stretch that was two notes beyond my capabilities. Several months later, I caught Dave Brubeck backstate at the Hollywood Bowl, and I just asked him to hold up his left hand. He laughed and said, "You've been trying to play {.....name of song.......}, I'll bet." He unfurled the biggest alien hand right out of the Schwarzenegger Total Recall movie and put it up next to mine. It was a good extra finger joint past mine in every direction. Holy Hell!
Thanks guys. 

I think I'll figure out on my own what needs to be copied exactly and what can be modified for personal preference! I think I am already learning what should and hsould not be done, now that I have the idea in mind of: 'some stuff you need to copy for long term development, and some stuff you don't - you'll learn what you can change by your own'.
Think ergonomically and logically.  Often there is no real reason why one fingering is chosen over another, especially for something like an E power chord, and especially in rock music in general. In the end it won't make much of a difference which fingering you choose since you'll know them all with time and will be able to choose the one that best suits each situation. 

Each fingering provides certain advantages - for example, playing the E power chord with your index frees up your ring finger and pinky to hit other notes up on the A string - a common blues technique ( think classic Chuck Berry type riff).  Playing the E power chord with  two fingers makes playing a  full E minor chord or an E minor 9 or 7 much easier, you can reach the upper notes on the upper strings with your pinky with that voicing. The trick is to pick the best voicing for what comes next.

For styles like Classical music, the specific  fingerings become much more important because of the level of difficulty and the intricacy of the pieces, but for blues and rock you can get away with quite a bit of variance.

 In the case above it, i.e. Marty playing the E,  it makes  zero  difference.

In the end, use your ears. 
Last edited by reverb66 at May 29, 2017,
I would try to learn things as much like the source material as possible. Some things do come down to preference, but you're not really at a level where you have your own preferences yet. Your preferences aren't really preferences if they're by default.

Many of these educational videos are going to show you things using the most "correct" technique possible. That's for two reasons: First, because beginners need to learn "correct" forms before they can make their own shortcuts, and second, because those correct forms are what beginners are most likely to be familiar with already.