There is a tendency to think that the often arbitrary rules of letter writing don’t really matter, that as long as your recipient can work out what you’re saying, that’s good enough. This shows a great deal of disrespect for your recipient, though, and for your own ideas. It suggests that a) your reader’s time isn’t valuable enough for you to write clearly and efficiently, and b) that you don’t particularly care about the content of your letter.
However, before using a general salutation (or leaving off a salutation), try to find out the name of the person you are contacting. If you are applying or inquiring about a job and the hiring manager’s name is not included on the job listing, you might look up the title of the employer or hiring manager on the company website. If there is a contact number, you might also call and ask an administrative assistant for the name of the hiring manager. If you are sending a different type of letter, you can still look up the person’s name on the company website, or speak to an administrative assistant or contact at the company for the name of the person you are trying to reach.