#1
I wonder if anyone has any tips or guidelines about when to change positions on the guitar fretboard?

Talking about single notes here, not chords

Now that I play 7 strings, I tend to cluster everything vertically into one position if I can, but I so often see guitar players moving all up and down the neck, and I can't really understand when it is a good time to do so.

For traditional 2 and 3 finger guitar players, I could see moving a lot, for sweeping arpeggios, I can see it making sense with some pretty easy rules (even then, being the lazy sod that I am, I may just do inversions to keep from moving). But what about for the four fingered guitar player?

I think what I'm tryin g to ask is for the closest thing to a set of if/then statements of when to move up or down the frets rather than across the strings. I'm not asking how to learn scales up and down the neck, I'm asking why I would play the exact same note in one position rather than another.

I see old surf bands moving their single note lines along with the chords that are being played, I wonder if that's part of the clue.
#2
There are two main reasons to move horizontally :

1) to change positions and have access to higher or lower notes.

2) the tone - playing something on certain strings has a certain tone to it - a note played on the E string does not sound the same as the same note played on the D string, despite it being the same note. Eric Johnson talks about this quite a bit in his video - he's super-obsessive about it generally. The effect is more drastic if your playing a note on a wound string versus the higher strings.

It's a great exercise to try and solo using just one or two strings - it changes how you approach things and it takes you away from mechanical patterns - John Abercrombie was a fan of that approach.
#3
Quote by reverb66
2) the tone - playing something on certain strings has a certain tone to it - a note played on the E string does not sound the same as the same note played on the D string, despite it being the same note.
this.

depending on the tone i want i will move vertically or horizontally. it really depends on what the song calls for.