Singular and plural words are differently recognized by search engines, there are many instances wherein a site would have good ranking for the singular form of a given word but would have bad ranking in its plural form. I personally think that it will be better if we're going to optimize for both forms using the most appropriate web page of our site (if it requires us). Dashes, apostrophes and other symbols can also give us different results and they can also affect our rankings.
Proper targeting of our keywords will play a very important role in our SEO campaign especially in our site's rankings. I strongly suggest to do this what we call "keyword analysis" first before doing off-page optimization techniques. We should also consider the competition and searched volume of our keywords.
That's it for now... GTG. Hope it helps.