1seo: Should I optimize for the plural or singular form of my keyword phrases?

What ever has more searches and what ever has less competitors. Decide on your own, you can optimize for both of course.

I suggest you to optimize both. Personally I even optimize mispellings since a typo can always appear when a regular user search google for a thing.
Just think as a potential visitor and not like a marketer and you will find your answer.