Personally, if someone IM'd me at work "good morning", I'd think that was a prelude to ask me a question, so I'd respond back to them. A "good night" would feel different to me and I likely wouldn't respond to that. I guess I feel like work IM's operate under a different set of social norms where you don't need to be so formal. But that's just me. If you want to stop sending them, I doubt she would think anything of it.