How to drink layered cocktails?
What is the socially acceptable tactic of consuming a layered cocktail?
That will generally depend on the type of layered cocktail one is drinking. There is generally no socially acceptable manner of consuming a layered cocktail! One is free to drink them in the manner befitting the individual.
A layered (or "stacked") drink, sometimes called a pousse-café, is a kind of cocktail in which the slightly different densities of various liqueurs are used to create an array of colored layers, typically two to seven. The specific gravity of the liquid ingredients increases from top to bottom. Liqueurs with the most dissolved sugar and the least alcohol are densest and are put at the bottom. These include fruit juices and cream liqueurs. Those with the least water and the most alcohol, such as rum with 75% alcohol by volume, are floated on top.
These drinks are made primarily for visual enjoyment rather than taste. They are sipped, sometimes through a silver straw, one liqueur at a time. The drink must be made and handled carefully to avoid mixing; however, some layered drinks, such as shooters, are generally drunk quickly. - Layered drink
As for myself, layered cocktails like the B-52 or a Black and Tan are consumed as shooters.
More complex layered cocktails, I use a stainless steel straw and carefully sip one liqueur or liquor at a time. To be honest, I enjoy the individual tastes of the different liqueurs. Some liqueurs have such an overpowering flavour that they are best drunk separately.
Chartreuse, Bénédictine and Absinthe are all strongly flavoured liqueurs and would overpower any layered cocktail if mixed all together before drinking them. Ergo, I use a straw!