I know where you are coming from.
The thing is, you should be looking at the equipment and determine if the reading matters. On the air lines and canisters, what impact would it have if the gauges got way off? Would it matter?
I know I (and some other keep saying that), but it's very difficult to answer a question about something needing to be calibrated or not without knowing more information. Every process is unique, and different.
For some processes, gauges on air/gas canisters are meaningless. They require no further action. For other processes, the readings are kind of important. Thus, they should be checked out.
For air, while the exact value may not be important, the indication of the presence of pressure is an important safety issue. You don't want someone messing with something that has 100 PSI of pressure on it. So, while the gauge reading is not important, the functionality of the gauge working correctly is important. You don't want a gauge to always read zero because something is broke, and the operator think there is no pressure, when in reality it's at full pressure.
You really need to do some process analysis prior to determining calibration requirements.
Talk to the people in your facility, ask them how they use the instruments. Ask them if they write it down, make some kind of assessment on it, etc. This will lead you to your answer of "does this need to be verified?"
