Recently took a two-day GIS overview class, which was basically a get-acquainted session with the GIS concept, the ESRI software and the available data bases.
Instructor made the point that people get into trouble sourcing their coordinate points from Google, as it doesn't use a proper projection system and therefore, doesn't match map data when imported. My takeaway is to double-check Google coordinates against another known source before relying on them.
My brain had glazed over by then (day 2, p.m.) so I didn't press him on the point, but hey, at least I remembered it!
To append my earlier comment--I plan my trips using GPS routes created in Topo and exported to my handheld. Route points are printed on the map. Time and again I've found the system invaluable for XC navigation, particulary through woods and over scree. Inevitably, when ascending to a pass in class 3 conditions the route and destination become obscured (convex slope, etc.) and one becomes preoccupied with safe routefinding across and through the boulders. Quick check of the nav screen bearing arrow and distance to the next waypoint tells me what I need to know to continue, while keeping at least one hand in contact.