The decision to add milk or sugar to tea is a matter of personal choice, though there is some debate about what is proper or creates the better tasting cup of tea. Many tea connoisseurs will argue that tea should never include milk or sugar. Some may even tell you that if you have to make these additions, the tea is low-quality and not worth drinking.
Beyond that level of snobbery, there are plenty of teas that are enjoyable with a splash of milk or a cube of sugar. Additionally, there are many traditions of adding milk, sugar, or both to tea, spanning all the way from England to Tibet.
How You Like It
When all is said and done, it should be a matter of personal taste. If you enjoy milk and sugar in your tea, by all means, add it. After all, you are the one who is drinking it, so any so-called "rules" of drinking tea are meaningless if you don't enjoy it.
That said, there are some teas that many people enjoy with milk and sugar while others are often best with no additions. Take these recommendations into account if you're wondering whether or not a cup of tea could be enhanced.
Whenever you're in doubt, the best thing you can do is to take a few sips of a new tea as it is before adding milk or sugar. If you find that you enjoyed it best unaltered, simply skip the additions the next time you brew that tea.
The Bolder, the Better
In general, bold, astringent black teas (or red teas, as they are known in China and Taiwan) will be your best bets for adding milk and sugar. This includes many single-origin teas from India (with the exception of Darjeeling), Sri Lanka (formerly Ceylon), Indonesia, and parts of Africa and South America. You may also enjoy either addition in any of the popular tea blends, including English breakfast and Earl Grey.
Earl Grey tea has as many preferences for drinking it as it has loyal fans. In England, it is often sweetened and a splash of lemon is added, though milk is rarely used. In the United States, however, it's very common to add milk. Then again, many people prefer it as is.
Some green teas, such as Gunpowder green tea, can benefit from a little sugar. However, white teas, oolong teas, pu-erh teas, and most green teas are rarely enhanced with sugar. There are several theories as to why. For example, many people think that white teas are too delicate and oolong is too complex for any enhancements.
Special Milk Teas
A great variety of tea recipes actually require milk and many of these are also sweetened. These fall into the category of milk teas and they are enjoyed around the world.
In England, Ireland, and Scotland, for instance, there are "builders teas." Developed by construction workers who couldn't take the time to brew a proper cup, these are known to be so strongly flavored that milk or a few sugar cubes are essential.
If you visit East Friesland in Germany, you're likely to encounter cream tea. Known as East Frisian tea, this is an espresso-sized serving of strongly brewed Assam tea with heavy cream and rock sugar. It is not to be confused with the "cream tea" of England, which is a version of afternoon tea, not an actual way of drinking it.
India has its famous masala chai tea with jaggery or sugar as a sweetener. Tibet and Nepal are known for tasty yak milk or butter teas. From Hong Kong, there is pantyhose milk tea with sweetened condensed milk and sweet bubble teas are a favorite of Taiwan. In North America, tea lattes are very popular and often sweetened with sugar or simple syrup.
Any of these milk tea recipes—and many more—can be made at home. Quite often, they are a little more complex than pouring milk or dropping a lump of sugar, though they're generally pretty easy.
Hold the Milk
There are also many teas which have are preferred with sugar but no milk. Iced tea is often sweetened, especially in the Southeastern United States.
Some tea-herb blends are also served sweetened. This includes Moroccan mint tea, which is made from boiled Gunpowder green tea, sugar, and spearmint leaves.