i would disagree with this. they are THE determining factor in 9 out of 10 things you want to do post-college, and grades follow you for LIFE.
the reason i started a company is because i didn't have the grades to do anything else except be a bottom-rung coder (not even at a "good" company), which I did for a few years after school. i graduated a very good UC with a 2.01 C average (i BARELY graduated, i was put on academic probation 3x, and took an entire quarter off at one point). i'm not kidding around when i say "i'm a terrible student" like some people do - i actually had D's and F's and plenty of W's on my transcript.
even with 5+ years of owning a successful business, if i were to apply to b-school or masters/phd program, they'd probably reject my ass. when was the last time you heard of a C student getting into a worthwhile grad school?