Worthwhile to put subreviewing on CV as a grad student? It is common for PhD students in Computer Science to assist their advisors with reviewing conference papers, known as subreviewing. The students' reviews may or may not get turned in with the actual reviews but it is a great form of practice for the students.

I have seen some students list the conferences they have subreviewed for in the Service section of their CV. I thought this was a little strange since it isn't official.

Should a grad student list this on their CV? It would help add some content to a short CV and show that they have at least some experience reviewing. It may also help a student associate with a conference that they haven't had the opportunity to publish at, yet.
The status of subreviewers, and even the designation as reviewer or subreviewer, varies a lot from venue to venue. In some places, reviewers are official and are listed in the proceedings. In others, it's as you say: informal assistance provided to the actual reviewer. 

A student should definitely list instances of official reviews (i.e where there's documentation of their effort) in their CV. But if the subreview is less formal (or there's no record of it maintained by the conference), then I'd be inclined to leave it off the CV, mainly because it's difficult to verify. I understand the desire to indicate some degree of competence with research critiques, but the danger is that without documentation the process is ripe for abuse or at least inflated claims.