Folks,
I removed the caliper and put the wheel on the axle spanning between two jack stands. I taped a chopstick and minimized the gap between the tip and the brake disc--essentially the smallest gap I can perceive with my 20/20 vision. Spinning the wheel, I saw no discernible change in gap size. I know this isn't as precise as dial indicators, but I really don't think that there is a significant warp in the disc. There was no issue with the disc before my steering stem bearing project, I never jimmied the wheel while it was off the bike, so it SHOULD still be good. During reassembly I decided to put the brake pads on opposite sides just for kicks. The problem persists--rattle/chatter at low speeds while moving the handlebars and a light but audible brake drag.
Could my piston not be returning for some reason? The fluid and dust seals looked good when I installed them, the piston is new... the piston certainly returns a LITTLE bit because when the brakes are off it feels like the brakes are off.... what else could cause a piston to LIGHTLY drag on the brakes?
I will scrub and clean the rotors like F64 suggested. I am also thinking of buying new brake pads. Anything to look out for there in terms of what supplier, materials, what is quality, and what isn't?