You definitely want to target singular and plural versions of your key phrases separately.
For a good example of the difference an "s" can make, you can spend some time at Google doing some comparative searches. If, for example you search for the word "apple" you get a completely different set of results than if you search for "apples."
The single word "apple" will produce a bunch of listings about the computer company, while the word "apples" produces a list of sites about the fruit. If you drill down to the 5th or 6th page of results in the "apple" search you will finally see a site about the fruit, but you will also see that Google doesn't highlight any ocurrence (in either the title/description or the cached page)of the word "apples." The site about the fruit is listed based only upon the ocurrence of the singular version.
Obviously this is somewhat of an extreme example because there are so many sites on the web that are about Apple computers. However, even if you do searches with two word phrases that are common to only one niche (something like apple recipe and apple recipes) you will see a similar pattern.
The trick is to figure out which version your target audience is most likely to search for, and then build your content around that version.