Coolant level will fluctuate when running. Mine would raise 1/4" when I released the pressure. However, extreme differences might indicate small bubbles (the air bubbles mixed in under pressure takes up less volume than under no pressure). This is what alerted me to pressure check my cooling system for HG leak... (about 1/2") and its a good thing, because I did find early evidence of failing gaskets. In my example, the dramatic increase in coolant level was probably both from aerated coolant and higher-than-normal system pressure.
Put all worries aside and do the infamous cooling system WOT pressure check. $15 is material and piece of mind (if you pass) or a head start (if it fails).