In the case of abbreviations, the quotes can be omitted, particularly if the abbreviation is well-known (i.e. not one that has just been made up).
The quotes are especially recommended when a new abbreviation is created or a capitalized common noun is defined for economy of text: when the reader sees it, he will recognize that the referent must have been defined at the beginning of the text, where the abbreviation or the capitalized common noun appears between quotes only the first time it is introduced.