This all depends on a number of factors:
a) do you want the synths to be prominent like an additional instrument, or to add another dimension/chordal flavour?
If so I would use synths in a higher octave than a guitar, and you can get away with using notes that don't exist in the guitar parts without it sounding overly clustered. I like to use synth parts with tight intervals that perhaps play a lot of 2nds, 7ths or 3rd in the chords - where the guitar parts would perhaps only be constructed using root notes and fifth.
b) Do you want the synth parts to be subtle and perhaps just fill out the sound?
If so you can get away with using parts in the same octave as the guitar parts, but I wouldn't stray too far away from the guitar parts in terms of the notes in the chord, other wise the music can sound too ambiguous/unclear in direction.
I don't think synth parts going LOWER than the rhythm guitars are very necessary, as they have to be quite loud for them to be heard properly, and it can muddy up the sound a little.
Personally, I like to layer about 6 layers of synths using different (but similar sounding) synth voices in a combination of both the above techniques... But then I'm more into creating a slightly ambiguous ambience, rather than big pompous and obvious synth parts.
Ultimately it's about context and what you're trying to achieve musically.