Questioning the need of machining rear discs to remove rust

I had problems with my rear (disc) brakes. I suspect it was due to misaligned piston to pad pin, when a body shop replaced or rear cross beam, but not sure.

Anyway, one rotor had an outside face with moderate scoring, and the other had an inside face where the innermost 50% of surface was making only light contact with the pad, and had gone rusty.

I got new pads and shims, also removed both rotors and hand cleaned them with steel wool, best I could. I did not get them machined. Apart from the scoring and rust, they did not exhibit any out-of-true pulsing.

So, assembled everything carefully, piston pattern oriented properly on pad pin, pressurized the pads to seat by multiple pedal tromps, then went for a drive.

They sounded pretty bad at first, making noise every time they were applied, and a bit of constant noise. In a few days of driving the noise died down. Months later now, they're perfect: healed themselves, with use, being properly seated.

I think shops will machine discs, at least sometimes, because the customer wants quick solutions, and there's some justification to that. The main downside, apart from expense, is your rotor's are thinning down faster.

But in your case, the discs in pic look very good, no reason to machine that I can see.