With all of the debate about turning rotors I thought I'd do a bit of
measuring and documenting while doing a brake job. Here is my results.

The car 1987(?) Honda accord; poor shape, 200,000K no brakes due to a
ruptured line. This soaked the pads and as per MAP standards required new
pads. Now disregarding the brake line (and the springs off in the right read
drum that he couldn't afford to repair) the car would appear as a prime
candidate for a DIY brake job in the front driveway. Both rotors had no
major grooves, very small ridge> I could picture this as a easy pad slap.

Now ASE standards have you measure several different angles to determine if
a re-cut is needed. We concentrate on the warpage factor. I don't feel like
buying a few hundred dollars more of measuring tools to tell me what one
pass on a lathe will tell me.

The right front rotor measured at .747 to start
   I took one pass at .002 on each side
   Final measurement was .742

The left front rotor measured at .697 on the outside edge and .703 at the
inside edge to start
   I took one pass at .002 on each side. the start of the cut was heavy, I
thought I'd have to do a second cut. Often we will see the O/S edge a
wee bit thinner than the inside edge, but when I returned to the lath one
cut was all that was needed.
   Final measurement was .696

I forgot to write it down, but discard was in the .660 range.

Most people who do brakes at home won't measure the rotors and it was
interesting to see the big difference in specs from the left to the right to
start. We thought there would be at least one new rotor needed, but specs
are specs.

You can see by the measurements that the lath action took off very little
metal, not enough to make a difference on heat dissipation, but the little
work guaranteed me that this car (the POS it was) had decent front brakes
when he left.
This is why I turn rotors every time. In the big picture, more metal is lost
in the time between brake jobs by the pads than on a simple clean up on a
lath.