Heating systems are closed loops, and do not normally pick up air over time.
Auto-fill valves are a solution-problem- they commonly fail/leak, allowing too much water in, raising the system pressure to rise. There is usually a shut-off valve between the auto-fill and the potable plumbing. When the system is set to the right pressure, close that valve. In the event of a system leak and LOW pressure you'll hear the sizzle & bang of the micro-boil inside the boiler going macr0 LONG before it's a safety issue.
As long as there's no air in the system, turn the fill valve completely off, and bleed the system down to about 12psi when it's not firing, and no pumps running. The turn up a thermostat a few degrees to start a burn cycle, an observe the pressure. If the expansion tank is pre-charged and sized correctly it'll rise a bit, but won't go much over 20psi even when the maximum system temperature is reached.
Most boilers come outfitted with 30 psi pressure relief valves, so there could be calibration drift on either the gauge or the valve, or it's possible yours was installed with a 50 psi PRV. If the calibration on the gauge is off you may hear some unusual sizzle at the boiler while it's running, in which case it's time to buy another permanent gauge, or a cheap gauge with a hose connection to check the system pressure with at one of the drain taps.
Fresh water corrodes heating system components and it's not a good idea to do it often, but it's also good practice to flush the crud out of the system once every year or two, normally at the end of a heating season.