Answer the question:  How precise do I have to be?
- Distances
 
- Altitudes/Latitude/Longitude
 
- Models (EGM, WGS, etc)?  Ellipsoids?
 
- Time Durations/Epochs
 
- Speeds/Accelerations
 
- Weight
 
- Angles
 
- Power
 
- Energy/Work Done
 
- Probabilities ([0,1] - [0,100], etc)
 
- Headings/Bearings
 
- Azimuth/Elevation
 
- Temperature
 
- Frequencies
 
- Force
 
- Currency