There is not much difference in the way you use your fretting hand physically on the M3 and on the six-string in conventional tuning. Since the fretboard on a 7-string is wider, and the number of semitones between adjacent strings on the M3 is smaller than on the six-string in conventional tuning you might think that you end up moving across the strings all the time. However, some types of runs are more comfortable when you move up and down the neck and, perhaps more importantly, many runs become easier to memorise and visualise when you stretch your fretting hand over more than four frets. A good example is the standard pentatonic scale which is spelled 1-2-3-5-6 in major. If you keep the 1-2-3 on the same string, spanning an interval of four semitones, you get an N-shape that is instantly recognizable. You might not want to use that N-shape very often (it is not terribly convenient to play, it has to be said) but even so you should be able to visualise it.