[Lil bastard]

There is a minumum wear thickness to the rotors, after which they should be replaced for top efficiency. This wear limit is 22.0 mm (Fr) and 18.0 mm (Rr) - Base or 26.0 mm (Fr) and 22.0 mm (Rr) - 'S'.

You use a micrometer (or a vernier caliper - though less accurate) to measure the disc on several points and average the readings.

If you don't have these tools, a good rule of thumb is that the rotors will last through 2 sets of pads and should be changed for the 3rd set - pads usually last 30-35k mi. under normal conditions, so at 20k or so miles, you should not need them. This does not apply if you are tracking the car, or especially if you use more agressive street or track pads which wear the rotors faster.

But, whether replacing them or not, in an absolute world, the rotors should at least be machined at each pad change. This eliminates any warpage which may have occurred and provides a clean base for the new pads to bed-in and lay down a layer of pad material on the rotor face (less important if you are using the same type pads as before).

If you are feeling any pulsation in the brake pedal, this could indicate some warpage. You'd measure this as Lateral Runout with a dial gauge and should not exceed 0.06 mm. This will be machined away if you have the rotors turned (machined) which in this neck of the woods costs about $15/rotor at a machine shop, usually while you wait.