Hey Rob, are you serious?
You can use every octave or key you like. As long as all the different sounds you use do not clash with eachother.
this is a strange question …
just use z and x in ableton to change octaves and play on your keyboard so you can get the tone in the octave you want/like.
There is no standard octave .
I do this… try making chords based in the key of your track (or not depending on how cheesy the music you make is 
)
Putting stuff an octave down… doubles up… and thickens from my experience. Just make sure to cut out whats not needed from the lower sound.
SO - Experiment with the same note an octave down… and also with chords.
I also lower the velocity of the lower notes & blend until it sounds the way I want.
Like others have said it varies from person to person as well as varying from sound to sound.
Its actually really good to think about what octave is used for what from start to finish cause a clean track cannot have too much sounds occupying the same frequency range. In fact at most I think it should be three sounds within the same frequency range on the condition you are doing things to move them away from each other (e.g. Widening/Note Placement). The less sounds within a certain range the better as it results in a cleaner track.
+1 Mistro.
Thats why less is more