From the guidelines:
> If the original title begins with a number or number + gratuitous adjective, we'd appreciate it if you'd crop it. E.g. translate "10 Ways To Do X" to "How To Do X," and "14 Amazing Ys" to "Ys." Exception: when the number is meaningful, e.g. "The 5 Platonic Solids."
> Otherwise please use the original title, unless it is misleading or linkbait.
The mods have been pretty good about making edits when appropriate.
I constantly see submissions here that do not use the original title. In fact, I have noticed that some/most of the time (I regularly save links from here in a personal db) the altered titles do a better job of summarizing the article than the original title. It happens to me all the time that I choose to save the link using HN's title instead of the original (better results when using it as tags).
If there is such a rule, it might merit more/finer discussion.