Depends on how much grain you actually add.
I often add grain, I think I set it to ~8 (OR it might be ~0.8, I don't have it in front of me right now) using the Premiere Pro noise & grain filter, only a very small amount, it's like adding salt to food; only add a pinch.
Something that happened to me, using CS6 I think, was that the video would just go crazy with artefacts everywhere when adding noise, I think it was only when adding it to After Effects comps within a Premiere Pro timeline that that was happening. Perhaps that is what happened to you?
I was using the CS6 trial at the time, I'm still on CS5.5, getting CC when I go back to College in September.