Here's a little something stolen from my last career as a cartographer / GIS analyst:
The reason the mountains are shaded as if the sun was shining from the northwest is because, due to history and at least in western cultures, our eyes are trained to read from top-left to bottom right. Therefore, when terrain is illuminated from the top left, it is quickly processed by our brains in the manner cartographers intend.
Try this: view a VFR Sectional (or any other like map) in an area that is familiar to you, except upside down to simulate as southern sun exposure (flatland folks.. sorry I cant help ya, find a new area to look at). If you do this, the valleys USUALLY appear as ridges and mountain ridges appear as valleys.
when I used to produce shaded terrain computer 'models', I used to put the sun at about 315 degrees, and about 60 degrees up from the horizon.
I hope this is the nerdiest thing I ever post on this forum