NSES Content Standard Unifying Concepts and Processes:  
Constancy, change, and measurement  
Grades K-12, page 118  

Different systems of measurement are used for different purposes. Scientists usually use the metric system. An important part of measurement is knowing when to use which system. For example, a meteorologist might use degrees Fahrenheit when reporting the weather to the public, but in writing scientific reports, the meteorologist would use degrees Celsius. 

Benchmark 12B Habits of Mind: Computation and Estimation
Grades 6-8, page 291
Determine what unit (such as seconds, square inches, or dollars per tankful) an answer should be expressed in from the units of the inputs to the calculation, and be able to convert compound units (such as yen per dollar into dollar per yen, or miles per hour into feet per second).