I would encourage students to support a cause that they believe in, and make a difference where they can.
I wouldn't advocate setting it up so all your incoming grad students get set up in the same outreach program. Thats just highschool all over again, where no one actually cares about waht they are doing, they are just going through the motions.
Finding outreach projects that fit my goals and interest me has been pretty rewarding. Encourage students to do it, yes. Encourage ALL grad students to apply for the NSF GRFP, yes. Give them prepackaged broader impacts activities? Hell no.