To show lack of consideration for those who in any capacity serve us — whether in restaurants, hotels, or stores, or in public places anywhere — is always an evidence of ill-breeding as well as inexcusable selfishness. It is only those who are afraid that someone may encroach upon their exceedingly insecure dignity who show neither courtesy nor consideration except to those whom they think it would be to their advantage to please.

Emily Post (1872-1960) American author, columnist [née Price]

Etiquette: The Blue Book of Social Usage, ch. 8 “Entertaining at a Restaurant” (1922; 1955 10th ed.)
    (Source)

See also Paul Eldridge.