Even if they can, you'd think they'd grow tired of it. After a hundred years of seeing everything and anything a person might do in private I'd imagine a ghost would no longer be interested in anything we'd consider off limits.
Everyone is so interested in talking to ghosts in order to connect with dead relatives, but if ghosts really see everything we do there may be far more practice applications to seances and EVP sessions. I don't think questions like "Did you see where I left my car keys?" or "What does my spouse do all day while I'm gone to work?" are out of line at all!