I am a non-expert, but did a google search for lineweight sketching, and line weight hierarchy. Found 3 rules here:

http://www.sketchmyworld.com/line-weight-importance-in-product-sketching/

Basic Line Weight Usage
1.The thinnest line is used to describe the edges of a product that point towards a viewer and also for the inner lines.
2.Second thickness lines are used for outlines (edges that have air behind them). These are not only contour lines, but can also occur within an object.
3.The thickest lines are used for the baselines – lines that show the interface between a product and a ground plane.


(Seems like there are a whole lot of drafting and architectural standards for line weight and line type. A person could
spend a lot of time on this subject, I guess.)

- Brian