Atmospheric pressure variations are of little importance with gauging pressures typically encountered in refrigeration and A/C service work. With a change of 2 inHg, the gauge changes by only 1 psi. It is unlikely that system pressures would cause concerns if the reading is correct within several psi.
Repeated pressurization (especially over-pressurization) of a gauge does, however, tend to change the response of the tube or diaphragm, and recalibration may be required.
The easiest way for a service technician to recalibrate a gauge is to connect it to a source of known, pure refrigerant, and then adjust the recalibration screw on the gauge to the appropriate pressure reading based on refrigerant temperature.
The most accurate way to recalibrate is to connect the gauge to a deadweight tester at a recalibration facility that has equipment traceable to NIST. This is usually cost-prohibitive for most situations, but may be warranted when total documentation is required.
Why should I zero out a manifold gauge?
As a general rule, a gauge can get off "0" due to a change in altitude or barometric pressure. It's important that you check to see that it's set back to "0" before you start charging. To reset a gauge, look for the reset screw that will be located on the front or back of the gauge. Then use a small blade screw driver to set the dial back to "0".