Which to use - the plural or singular form of your keyword phrase?

I don't know how you do your competition research, but I've been using a program called Traffic Travis which does all of that for me and then tells me how difficult it will be to rank for each keyword. It will say if a keyword is Easy, Relatively Easy, Difficult, Medium Difficult, Very Difficult. I don't pay any attention to what the google keyword tool says about competition because sometimes even when it shows high competition you can still rank for the keyword - Traffic Travis will show me that it is Easy to rank for. I believe Traffic Travis looks at the top ten pages in the search results for a keyword and looks at a number of factors like their page rank, number of backlinks, if keywords are in the title, etc. I'm not sure what else it looks for. But, in any case, sometimes it will show that both of the forms (plural and singular) of a keyword phrase are Easy to rank for. And that leaves me wondering which one to use.

Basically, I'm just wondering if you can also rank well for the singular form of a keyword if you use the plural form, or if search engines just show the form that the person searched for first. Because sometimes they are both Easy to rank for but the singular form of the keyword has more monthly searches. Obviously if they only show the form the person is searching for first, you're going to want to go with the form that has the highest monthly search volume unless it doesn't read well in your article. But, if you can also rank for the singular form when using the plural form of the keyword that would be even better.