As a sign man for nearly 46 years, give or take a year or two (who can remember that far back?), the only time intelligent businesses use quotation marks in the USA in their signage is when it has been created, as a slogan, through newspaper, radio, or TV advertising. I always advise not using quotation marks if the quote is not well known, and if it has not been accepted through the media. Otherwise, it it will not be taken seriously.
So far, all my clients have agreed.