The disc shouldn't touch at all, so any sound indicating contact is contra-indicated. OTOH - friction is a function of the material characteristics multiplied by the normal (perpendicular) force pushing the materials together. In the case of incidental rotor contact characterized by an occasional chirp, there's no significant contact force, so not much friction at all.
One experiment you might do is spin the wheel slowly and let it come to rest. mark the position, and repeat a number of times, marking where it stops each time. If there's enough friction to worry about, you'll find the wheel tends to stop in the same area a large percentage of the time. Otherwise, if there's no pattern, then there's no need to fret.
Of course, nothing should stop you from trying to eliminate ALL incidental contact, so I'm only talking about those situations where you can't seem to, ie. a slightly bent rotor, or a flexy frame.
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