It depends to whom the letter is being written. If you are writing to a network executive, or someone with a network, studio, or production company title, it is appropriate to send a business style letter. Therefore, the top left a few spaces below the date, will have the person's name, under that the title, and under that the address.
Depending on the nature of the letter you would address the recipient as Mr/Ms/Miss/Mrs or by their first name only.
When you are writing a letter to someone whose title is director of photography, it's a little formal to put that in a letter, HOWEVER, if they are in the ASC or another honorary organization, it is with respect, that you would include their membership letters after their name.
When deciding about including a title, think about what you would wear to an interview. Most crew members who interview in a business suit, "freak out" people because camera assistants don't wear business suits. So if you're a camera assistant writing to a camera operator, you probably wouldn't use his/her title, but you would put SOC after his/her name if it applies.
