Yes, I find that your chances of being assigned a middle are no better or worse than being assigned any other seat. One thing I have noticed, now that I think about it, is that one the MD-80s, I have never been assigned A or C seats on the left side of the aircraft when checking in. I don't know if that's just because they get grabbed by couples flying together but I don't recount that ever happening. Perhaps I'm just an anomaly. I don't recount the same thing on the 757s or A319s.

TLDR: They fill back to front, so your chances are no better of getting a middle seat than any other unless there are multiple middle seats available towards the back. In that case, just wait until those get filled in before checking in.