Dear Sirs or To Whom It May Concern. While it may have worked for you in the past, it is far better to do the legwork, find a name, and submit it to the right person. Why? Simply, because any and all employers see this generalized introduction as lazy and standard. They are looking for someone who can do great things for their company and the position, and want someone who can show initiative and enthusiasm. Finding the correct address for the cover letter is one of many little examples of this in an applicant.
So, if there is no specified address, how do you find out this information? This can be found out by a number of different ways. First, it is wise to contact the human resources department in that company and ask them directly who to address the application to. If the company does not have a human resources department, you can call their general administration and inquire. If the company is difficult to get in touch with or if they specified not to call in regards to this position; do your research online through the Internet, library, and their online site to find department heads and hiring managers listed in some sort of directory.