Obsessive Compulsive Barkeeping

In fiction, the sole Idle Animation for bartenders who aren't currently filling a glass is to dry a glass (unless they're the main character, in which case they will never do either of these things). You'll never see them stocking ice or frequently-ordered drinks, or trying to keep customers (who aren't the protagonist) engaged. In real life, bartenders who always have a glass in their hand tend to get fired.

Understandable in TV and Movies, where its an easy barkeeping duty to do in multiple takes, while many other jobs would need undoing before they could be done again and cause continuity problems. Less understandable in other media; it's probably a result of the stereotype.

Anime and Manga

 * Giriko Kutsuzawa in Bleach, specially in the anime.

Film

 * The bartender in Blazing Saddles who "always kept things nice and clean" (by spitting in them and shining them.
 * The bartender at the saloon in Back to The Future Part III.

Literature

 * In Harry Potter, Aberforth Dumbledore's first appearance involved him wiping a glass that seemed to be getting dirtier.
 * The barkeep washing glasses is an obvious cover for his eavesdropping. Two books later, the stereotype falls away altogether as he becomes a full-blown Rick Blaine type.
 * Lampshaded in Guards Guards. Sham the bartender is constantly cleaning glasses, which always confuses Vimes because the glasses never seem to be clean.
 * A possible (and obvious) answer to TP's dilemma offered by - who else? - Douglas Adams is that the cloth is actually dirtier than the glasses, resulting in an opposite effect where the alcohol kills whatever germs have peacefully been living on the towel.
 * Mac, the proprietor of McAnally's Pub in The Dresden Files, is always either wiping glasses or polishing the bar.

Tabletop RPG

 * Dungeons & Dragons adventure I6 Ravenloft. The barkeep of the "Blood of the Vine" tavern mindlessly cleans glasses one after the other. When they're all clean, he starts over. He only stops doing this to take drink orders and serve drinks.

Video Games

 * Brewster the bartending rooster in Animal Crossing.
 * The bartender in Paper Mario: The Thousand-Year Door is doing this.
 * The bartender in Monkey Island 2: LeChuck's Revenge is always cleaning the glasses... with his spittle.
 * The bartender in Jedi Knight II invokes this, cleaning a glass as an excuse to ignore Kyle Katarn. Once Kyle slams his lightsaber on the bartop he tosses it over his shoulder and switches rapidly to obsequious mode.

Western Animation

 * Moe Szyslak does this all the time on The Simpsons.