First check out How do I write a good title? on the global meta site. It includes:
- Keep it short
- Lead with the most important words
- Don't start with "How do I"
What titles shouldn't be
I would go with "neither". While it may initially sound like a good idea to make everything a question, it's ultimately misguided and counterproductive. Forcing titles to be questions forces the inclusion of boilerplate text. We want to avoid boilerplate for the same reasons that the word "question" isn't allowed in titles and "For issues relating to" isn't allowed in wiki excerpts, and why salutations are stripped. The title is essentially the elevator pitch for the question. When skimming titles, especially in the "Linked" or "Related" sidebars, people want to easily tell what differentiates those questions so they know whether it really is related to their own problem. If they all start with "How can I", "How should I", "How do I", then that makes it harder to tell them apart, not easier.
What titles should be
A question's title needs to be detailed, but succinct. Consider the titles of news articles or research papers. They need to convey the main thrust while fitting in a limited space. I wouldn't go so far as to change all titles that are questions to not be questions, but the added boiler plate is a hindrance to the goal of the title: to distinguish one question from others on similar subjects.
To borrow from my answer in The Great Debate:
What we really want is for the title to be the well-described subject
(in the grammatical sense) of that question which starts with "I need help to ...".
For the example given, without more detail, I'd call it "Painting a ceiling". Though I'd much prefer something like "Applying oil-based paint to a textured drywall ceiling". Any words that can be omitted from the title without obscuring what the question is about, should be.
Effects on searching
Regarding search optimization, if all questions start with some variation of "how do I" then those words become useless as search keywords. Other Stack Exchange sites have already run into this problem and requested that these words be ignored by the site search. Google will also have figured this out, as they do everything they can to ensure that pages' ranks are determined by their usefulness rather than SEO games. Also, as Jeff Atwood points out: "Google only supports a fixed # of words in the title of a page? Do you really want those eaten up by boilerplate?"
Compare these titles, written as questions, then as summaries. The question form takes much longer to scan through, without improving your understanding of what you'll find when you click the link at all. If you're wondering about replacing a shower door, which style allows you to determine which related questions are relevant more quickly?