Have you ever read and responded to a letter that greeted you with Dear Sir or Madam? We're going to go ahead and guess you haven't. Not only is this salutation stiff and formal, it shows that you couldn't be bothered to look up a contact name and address someone specific.
The same sentiments that apply to Dear Sir or Madam apply here. If your letter opens with To Whom It May Concern, we're probably going to assume it doesn't concern us.
Don't use this greeting with job application cover letters. Make a point to find the hiring manager's name, even if that means calling the company and asking. If you can't find a name, "Dear Hiring Manager:"or "Dear [Company] Team:"will work.