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There is a very good test, to know when you've clipped the seam allowance of a curve enough, so that it will lay flat, smooth, and perfectly curved when done. The test is whether the stitching line can be held out straight. The link above shows images of this test.
The greater the curve, the closer you need to make each clip to the next clip, and the deeper the clip needs to be [closer to the stitching].
An example of this is a neckline with a facing. The back is only slightly curved, so it only needs to be clipped a little and not very deeply. The front is usually curved a lot more, so it needs to be clipped more.