From Estibot.com:
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restaurants.com: Appraisal USD 2,200,000 *
restaurant.com: Appraisal USD 1,500,000
hotels.com: Appraisal USD 11,000,000 *
hotel.com: Appraisal USD 6,600,000
girls.com: Appraisal USD 6,200,000 *
girl.com: Appraisal USD 2,200,000
cars.com: Appraisal USD 2,500,000
car.com: Appraisal USD 3,100,000 *
There is going to be many opinions about this and I don't think anyone can give you an answer that is true for every domain or even most of them. But in terms of SEO, it will make a difference, but mostly based on the number of searches each version of the keyword gets. And the plural version has a better potential for matching searches for the singular and plural versions, where the singular version may not turn up at all for the plural version searches.