The conclusion in this meta SO question was that sentence casing is preferred. I agree.
Am I now encouraged to edit titles that use title casing? (Or no casing, or all caps, etc...)
You are encouraged to edit posts that need editing.
Ask yourself: does the title really need that edit? Are you adding value to it? Are there other problems with the post that you could fix at the same time? Or is it simply triggering the mild form of OCD that a lot of programmers tend to have?
There are a lot of horrific titles out there. Personally I don't edit them unless it makes the title clearer, less ambiguous, easier to read, corrects typos, etc. All lower case is ugly and you can fix it, but your job will never be finished.