Like everything else that requires routine maintenance for optimal performance, spectrophotometers are no different. Spectrophotometers are crucial tools for color workflow management and need to be calibrated regularly in order to give consistent and repeatable results.
Most hand held spectro units ship with a calibration plate that is used specifically for the calibration process. Since each device is shipped from the factory calibrated to this plate, you should not mix calibration plates between two similar instruments.
If you rely on your device for critical measurements, re-certification should be considered as spectrophotometers will drift as the light source and integrating sphere ages. We recommend re-certification roughly every year. This process involves sending the device back to the manufacturer and can be somewhat expensive, but worth the cost when critical measurements are a must.