As a counterexample, I had terrible grades and dropped out of university in my second semester. I've never once had anyone ask me about grades or about my schooling in a professional context, and I've worked at some pretty fantastic companies. I can't speak about grad school, as I know nothing about academia, but a significant proportion of the best technical professionals I've worked with didn't graduate university; I've even worked with a few that didn't graduate high school, but still work for top tech companies.

A referral from an employee who knows that you are competent and reliable is worth more in the hiring process than anything you could list on your CV.