Worthwhile to put subreviewing on CV as a grad student?
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.