Use "couple of" if you assume couple to be a noun, such as pair: "a couple of apples" or "a pair of apples." Drop the "of" if you assume "couple" to be an adjective, such as few: "a couple apples" or "a few apples."

Why is "couple of X" grammatically correct? What is the logic? And if "of" should be inserted after couple, then why not after "few or "several?" A few of things? A several of things? "A couple of things" sounds pretentious to me -- "of" is an extra word that has no use. I say drop it.