Graham Cox has sent me some photos of a JR Eventide, "Tulka", in Queensland. I've put one into the Box, 'miscellaneous photos' folder, because it illustrates a point perfectly. The sail is a standard Hasler that has been cut flat, with a dead straight head. The yard is bending a bit, as it must, with the result that the luff and leech are going slack, and the centre of the sail is taut, to the detriment of both its performance and its longevity - the tablings will flutter and tend to destroy themselves.
By all means have a flat-cut sail, if it suits your sailing style, but please - round the head of the sail as much as the yard is going to bend, and then a little more. And it wouldn't hurt to put just a little rounding on the seams of the top two triangles either.
In order to end up with a flat sail, you have to put in some curves.