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When you import a texture to Unity, you are setting up some of the properties that sampler2D will use. The most important is the Filter mode, which determines how colors are interpolated when the texture is sampled. It is very unlikely that the UV data will point exactly to the center of a pixel; in all other cases, you might want to interpolate between the closest pixels to get a more uniform color. The following is a screenshot of the Inspector tab of an example texture:

For most applications, Bilinear provides an inexpensive, yet effective, way to smooth the texture. If you are creating a 2D game, however, Bilinear might produce blurred tiles. In this case, you can use Point to remove any interpolation from the texture sampling.

When a texture is seen from a steep angle, texture sampling is likely to produce visually unpleasant artifacts. You can reduce them by setting the Aniso Level to a higher value. This is particularly useful for floor and ceiling textures, where glitches can break the illusion of continuity.