I'd say that all objects in space cast a mass shadow into hyperspace. Pirates even use asteroids which they haul into hyperlanes in order to trigger the failsafes (poor man's interdictor).
As I understand it, the art of astrogation is plotting a course that doesn't hit any of them. While a nav computer operates like a GSP device does in ours, anyone who has relied on GPS devices too much has also learned that they're not infallible. The skill of astrogation lies in knowing how not to trust the nav computer - what corners you can cut and which you can't.
Either Han and/or Chewie (and my money's on Chewie) are excellent astrogators and the Falcon has a great nav computer, which is how they made the Kessel Run in 12 parsecs (or so Han alleges).