The first thing ANY employer, biglaw or non biglaw will look at is grades. Biglaw just happens to be more
stuck up open about it and I'm pretty sure they have cut offs unless you really know someone or something amazing happens. "Regular" law firms will look to that first thing on your resume. Even public interest is competitive. Hell, any legal jobs for attorneys are competitive, and even moreso in this economy. Grades matter anywhere.
I would say the sort of clear area you can possibly escape from this is if you do public interest and show a clear dedication to "the cause". If you want to be a domestic violence advocate, for example, showing you've been working in a clinic for three years, is going to be more impressive than your grades. If it's you vs. someone with better grades, they'll take you because they know you're committed to the cause. That being said, they would also like you to have good grades. I know the public defenders program is also less picky when it comes to grades, because, again they would value commitment over grades.
They always matter, just the more "high ranking" or competitive a job you're seeking, the more important they become. There are always ways to overcome grades, or at least can mitigate them - good interview skills, experience, networking, extracurriculars (if you founded something or did something impressive, not just being in them).