The general rule of thumb is that the resume and cover letter you submit for any position should be crafted for that particular application.
That doesn't mean that you should rewrite the whole thing, but definitely try to change a few key phrases in order to target that particular employer. It will show the person reading your letter that you put in some effort to apply for their position, and it will also improve your chances.
Consider that quite often the person reading your resume/cover letter is not an expert, but rather an HR drone playing buzz-word bingo with your application.
Is he "enthusiastic"? Is he a "team player"? Does he give an example of programming a doohickey? What about sheering a sheep? He did? Great! The manager can take a look at his resume next!
Let's say that this company is looking for someone with great "enthusiasm". If your original letter reads something like:
To Whom It May Concern,
I'd like to submit my application for the BLA-BLA-BLA position your company has advertised. (fluff).
I believe that my experience sheering sheep in Australia last summer makes me a perfect candidate, etc.
I look forward to hearing from you, etc.
You might change it to:
Dear Human Resources Professional,
It is with great enthusiasm that I submit my application for the BLA-BLA-BLA position your company has advertised. (same fluff).
I believe that my experience sheering sheep in Australia last summer makes me a perfect candidate, etc.
I await your reply with great anticipation, etc.
See what I did there?