Checking in on-line, in my experience, doesn't affect the chances of op-ups. A lot of op-ups are given at the gate anyway.

Not sure if your seat going unallocated means anything though. Some people seem to think it means you might be getting an op-up, but then other times when you go and check-in you get your old seat back again. Am sure others on here will have experienced that though and will be able to comment.