Depending on the configuration of your process and how important that specific measure is for your performance, some flow meters should be calibrated whereas others can be left uncalibrated.
Usually, flow meters part of a secondary loop system are redundant as any drift in the measure will be corrected by the action of a primary loop or an operator that makes set point adjustments based on some other process input. This is true for most flow meters in circuits that act as an intermediate loop for a given process. These loops are generally measured by some other equipment and they are unlikely to require absolute flow measurement accuracy. It is usually enough for flow meters to offer stable, consistent performance in the short term for this type of configuration.
Flow meters that are likely to require routine calibration include those impacting product and feedstock quality and quantity, energy/fuel quantity and critical process operating points.