No need for the singular word as the SE's automatically parse it from the plural but you might want to consider using long tail key phrases instead of isolated keywords. This would allow you to target search phrases with a high degree of precision while still using the singular and plural versions of the keyword.
In truth, you'd most likely see more benefit by ensuring that your page title, anchor text, headings and content use both forms.
Here's a link to a pretty great article that may be of use:
Hitwise Intelligence - Robin Goad - UK: Plural or Singular search terms?
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