Very important question, and I really didn't know the answer until I started reading. Main conclusions:

  1. There is some connection. If the plural form exists and is relevant, then it's imprtant most of the time.
  2. The position of your app in using Singular keywords will be different from using Plural, most times.
  3. An exact match of keyword really counts, so pic the form that will generate the highest amount of dowenloads and higher position in the search results.
  4. For English, using Singular is usually better and safer.

That's just a part of it. You can read it all here:
http://info.localytics.com/blog/singular-and-plural-keywords-app-store-optimization