We prefer to moderate post rather then locking a topic.
Our normal policy is , just as you ask, if we see a thread that needs to be locked , we lock it and don't do further moderation unless it is copy write/porn excetera.
But what happens many times is we start reading a thread, start moderating a few post in the thread, and then later decide it's time for the entire thread to be locked. But at that time we the posts have already been moderated.
HiTech