I wouldn't worry about it. Any company that requires open source contributions hasn't thought very hard about their hiring process. They would be excluding the vast majority of talented (and especially professional) programmers.

It's probably most useful as a bullet point to attract a recruiter's attention. So commit a line of code somewhere and stick it on your resume.

Not to say you shouldn't work on an open source project. But do it because it's something that interests you, not for your resume.