Hi Yoong Liat,
I'd hesitate to say simply 'if you do not know the name of the person you are writing to', because the normal way to open such a letter would be to say 'Dear Sir', or 'Dear Sir or Madam'.
There is so much discussion on the Forum about 'To whom it may concern' that I think people do not realize that this is used very, very seldom and only in very special cases.
I don't remember the last time in my life that I wrote a letter that started in this way.
Best wishes, Clive