We're not totally sure exactly what the right answer to this is yet, but here are some things to think about:
The most important thing that topics are used for is distribution. A good way to decide if it makes sense to add a topic to a question is "Would people who signed up to monitor questions about topic X want to see this question?" Adding a topic to a question causes that question to show up in the home page feed of everyone following that topic (the details of this may change over time, but in general, following a topic is sort of like subscribing to questions associated with that topic.)
There are some other less important considerations that you might want to think about.
– Clarification: Adding topics to a question can help other editors clarify the question text and give good answers. We're still trying to figure out whether a question should be defined entirely by the text of the question or the tuple (question text, question details) or whether topics should be part of the definition of the question. So, in general, its probably a good idea to add a topic to a question if it helps others understand your question better.
– Expertise and topic-specific bios: When we show answers that others give, we'll show a topic-specific bio for anyone who has one on a topic associated with the question. In addition to just showing the topic-specific bio, we'll probably at some point give more weight to answers that come from people who've demonstrated expertise in a given topic area. So, if you ask a question like Where is the best sushi in Palo Alto? then it could make sense to add the topic Sushi to the question since, if a famous sushi chef visited Palo Alto and surveyed the restaurants there, he might have an opinion, and if he answered the question, we'd want to take into account that he really knew what he was talking about (with regards to sushi,) and we'd also want to be able to convey that through his topic-specific-bio (ex. "Sushi Chef for 43 years, Owner of Shiro's Sushi in Seattle until 2007, etc.") So, in that case, a set of topics like Sushi, Palo Alto, CA ,Restaurants in Palo Alto seems reasonable.
Another thing to think about is that it depends on the number of people who are signed up to follow each topic and how deep each topic area is. For example, if it were early 2005, it might make sense to have just one topic for Facebook (product) because it was just a little website with a few thousand users, but now it makes sense to have different topics for Facebook Platform , the Facebook API , Facebook (company) , etc. I don't think there's an a priori way to determine what level of specificity makes sense for a given question; it will generally be a good idea to take into account how much attention is focused on the topics that could logically make sense on the question.
In general, it probably makes sense to add a number of topics that encompass a range of specificity and also lean towards adding more topics rather than fewer if you're having trouble deciding.