Many people wonder if there's any difference between wet and dry measuring cups. There is.
A wet measuring cup has the line a little below the top of the cup, so you can avoid spillage. Tapping a wet measuring cup to settle a dry ingredient to the desired line will cause some fine ingredients, like flour or sugar, to compact. This can wreak havoc with fragile recipes. For example, an extra 10 grams of sugar in certain cake recipes would be disastrous.
A dry measuring cup filled to the top with a liquid will yield a roughly correct measurement, though pouring it may be messier without a spout.
For exact results, San Francisco baker Rachel Leising recommends using wet measuring cups for liquids and a scale for dry ones. Note that each dry ingredient will have a different weight: One cup of sugar does not weigh the same as one cup of rice. Several online resources, such as GourmetSleuth.com, provide measurement conversions for commonly used dry and wet ingredients.