Final answer:
It is true that only the first word of a complimentary close should be capitalized when ending a letter. Proper letter format differs from title formatting where important words along with the first and last are capitalized. Proper nouns and proper adjectives within a sentence should also always be capitalized.
Step-by-step explanation:
The statement that you should capitalize the first word in the complimentary close, but not capitalize the second and following words, is true. When you are closing your letter, you should start with a capitalized word such as 'Sincerely' or 'Best regards,' followed by a comma, and then leave two blank lines before typing your name. This is in line with conventional business letter formatting.
In contrast, when you are formulating a title, capitalize the first and last words as well as all major words in between. However, do not capitalize articles, coordinating conjunctions, or prepositions unless they are the first or last words of the title or subtitle.
Use of capital letters is also advised for the first word of a sentence, proper nouns and proper adjectives.