Should I listen to quarterly earnings calls for all 20+ companies on my watch list?

Ideally, but of course that's not always realistic.  Because I'm lazy, I listen to the ones that I'm likely to buy in the near term -- when the industry group is getting good, i.e., when the money is headed that way.  This keeps the load down a bit.  But if you have time and inclination, it's best to keep up with all of them.