When should spoilers be permitted in titles?
The rule I've always followed is to include spoilers in your title only if it's required for the title to be searchable, and clearly indicate what the question is about. In other words, if you can make a good title without spoilers, do so; otherwise include spoilers. A good title is more important than spoilers. After all, the ignore tag feature will allow users to ignore any tags in a series they wish to avoid spoilers about.
When should we require users to include spoiler markdown in the question body?
Spoilers should be used for events beyond what the scope of the title suggests.
When should we require users to include spoiler markdown in answers?
Spoilers should be used when discussing events or theories which contain information revealed later in the series than the question. For example, if I ask "Why does Joe kill Bob in Chapter 3?", but this isn't answered until chapter 15, the answer should make heavy user of spoiler markdown (ideally you'd want to format the answer as "This is revealed in chapter 15: spoilers go here)
What do we consider a spoiler?
This is the tricky part, since everyone has a different bar for when they consider something to be a spoiler, and is something which I think we'll develop over time as a community. For a start, we can obviously classify major events as spoilers. Outside of that though, I'm not really sure. I don't think we'll be able to come up with hard and fast rules for this, and is something we'll need to handle more on a case-by-case basis as opposed to have a set of rules typed up which capture every situation.
Should we make use of a spoiler tag?
No. A spoiler tag suffers from a few problems.
First, it takes up tag space. We can only have 5 tags on a question, which may seem like a lot, but do you really want to give up a fifth tag that one time you Really need it because you need to include a spoiler tag?
Second, the sites method for determining which tag shows up in the title of the page is decided by popularity. Do we really want spoiler to show up in the page title over other tags like. Since a spoiler tag is generic it will be among one of the more popular tags on the site, and likely to be the top tag on most questions where it used.
It's also not useful as a filtering mechanism, since no one is going to ignore all tags with spoiler. It just wouldn't work. After all, I don't want to ignore questions with spoiler, I want to ignore questions from specific series. The spoiler ignore would just hide questions from series I want to see.
The tag itself also doesn't reveal to a user whether or not it's a spoiler for them. If a title is ambiguous so as to not to reveal what the spoiler is, then how does adding a spoiler tag reveal what it is? If it's a spoiler for a portion of the story I have yet to reach, then how will the tag let me know that? All I know is that there is a spoiler of some kind in the question, but the title should be enough for me to realize this. If that isn't the case, then tossing in the tag doesn't help. Making proper use of spoiler mark down would be considerably more useful for revealing when and where the spoilers take place, and if it's a question I should avoid reading without actually spoiling anything for me.
Finally, it will be overused to the point of being useless. Considering the nature of our site the vast majority of our questions will contain some type of spoiler, making the tag required on all questions. Do we really want that? I think not.
Should we include the word "spoilers" in the title?"
Including the word "Spoiler" in a title is a bad idea. It makes the title less natural, and just creates clutter. Titles should be clear, and concise. A similar way to look at this is from searching. No one is going to search "SPOILER What happens at the end of chapter 15?". If no one is going to search it, then does it really need to be in the title?
It also suffers from the same problem that the spoiler tag does, which is that it becomes pointless due to overuse. If most titles have spoiler at the start, I'm eventually going to just start ignoring the word all together.