Seriously, save it for writing a formal business letter or email. There are better alternatives for Dear Sir or Madam that will help you remain professional.
The first thing I think when I see this you didn't even attempt to check out my profile to get to know me. At least if you address me as Sir, I'll be slightly less offended because you got the right gender down. However, in today's technologically connected world, there is (almost) no excuse for not knowing whom you are writing to.
First impressions are important and this opening always seems to get my attention for the wrong reasons. How about you?