The rule is, as far as I'm aware: Anarchy systems/stations always WILL have Interstellar Factors. Low security low population stations (mostly outposts) *may* have interstellar factors. The GalMap by itself is not always accurate. I don't think it will give you false positives but it will give you false negatives, so you may end up traveling further than you need to.
Galmap shows you IF where you have been earlier.