Nope. If the caliper is post mount, it's likely on an IS adapter. Have you loosened the bolts which join the two and tried re-aligning it?

If that doesn't work (e.g. there's not enough adjustment) you might need to fit some shims between the frame and the adapter.

If your frame has a rear post-mount fitting ignore this (what frame is it though?)

Is that really true? I have post-mount callipers on an IS adaptor, but they won't line-up perfectly. There's no way I can see of getting the calliper slot to be perfectly parallel with the rotor, unless I have the mounts faced.

That's what the shims are for.

But how would that work? The bolts allow for fore & aft adjustment, but they don't allow adjustment for the angle of the calliper relative to the rotor - i.e. so that the rotor is perfectly parallel in the calliper slot from top to bottom. Surely the only way you could alter that is if there was some way of rotating the calliper?

If the caliper won't sit parallel to the disc in a vertical plane then yes, get the mounts faced and then add a couple of shims(most adapters/mounts have a flat face so will need spacing to sit correctly). Also be aware that some adapters aren't actually square, and despite facing the mounts the caliper will still sit at an angle, careful use of a file on the adapter will see it sorted as a last resort.

(without reading the entire thread) isn't this what the dome head washers (as seen on avid) are for?