The commonality of symbol itself isn’t necessarily the point. If you have one that is easily identifiable (such as the anarchy sybmol) and the location is readily apparent, say on your left arm on your elbow, then that’s enough for them to go with to help with ID the person. While there may be plenty of the symbol around, the odds of a person of a certain age/sex/color having that same symbol in the same spot is pretty close to zero.

Late edit: if you want to fool visual identification via tattoos, get fake tattoos that are visible and then remove them.