The issue is controversial. We have used graphene modified electrodes in bioelectrochemical systems (bacterial communities attached to electrodes) and it seems to foster biomass development. If you're interested the article is "Graphene oxide electrodeposited electrode enhances start-up and selective enrichment of exoelectrogens in bioelectrochemical systems" and it's freely available in my RG profile or as open access in Scientific Reports site.
You were not specific about the type of cells you intend to test so it is difficult to say much. Overall though, cytoxicity results seem to vary a lot with the physicochemical properties of GO/graphene, as well as how the experiments are set up. A few studies pre-treating GO have shown lack of toxicity. You would have to carry out the test in your own system to know for sure. My advice is to also probe for sub-lethal effects.