I've always hated when mods (and higher staff) lock threads that don't need to be locked.
I think locking should be a last resort. The only threads that need to be locked are threads that need absolutely no responses. So rules, the thread approval updates that are found in moderated forums, informational threads like the thread about the chatroom, and other things like that.
If a thread is in the wrong place, move it to the right place. If there's already a thread about a topic, merge it with the existing thread. If a thread already fulfilled its purpose, you really don't need to lock it. If someone has another question about something relating to the topic, they should be able to use that thread already existing. Your efforts aren't as well noticed that way, but they're more appreciated.
If a thread gets out of hand, and the moderator's tried to bring it back into the topic at hand to no luck, and people are just getting carried away, then I think that's the only time a thread needs to be locked.
Unfortunately, all moderators do things differently. But this was how I always advised new moderators to handle threads when I was on staff.