Use double quotes for direct quotations only. A direct quote is something that someone actually said or wrote. Vernacular, such as hillbilly and hushpuppy, are not quotes and should not be emphasized without good reason. If they do not stand on their own without emphasis, they should be replaced with more appropriate words.
Use single quotes for internal quotations (quotes inside other quotes). If you are British, you may reverse that — the only permissible exception in formal writing.
If you absolutely cannot help yourself, use italics for emphasis. Generally speaking, it is best to let the reader find her own emphasis in your writing. In any case, don’t use “quotes” for ’emphasis’ as I just did — ever.
Go now, and sin no more.