As Rick pointed out early on, this is indeed a personal matter. Not only in regard to shape, but as to how important a factor it is. As there is no one-fits-all answer, a builder is alway left with an imperfect solution.
Nonetheless, we like our method, of course, or we wouldn't be doing it.
I'm not a big fan of radiused fretboards - of course that's personal again. Still, the natural shape of most people's fingers is not a "C", but something closer to a "J" (one that doesn't curve up very much). To me, the upper strings are actually a little more difficult to play on a standard radius.
Everyone assumes a classical fretboard is flat. So did I. As a matter of fact, early on in my association with my partner Omar, I thought that's what he was doing. His background, is high-end concert classical guitar, and our fretboards looked flat at first glance.
When I asked him one day about his flat fretboards, his reaction was shock. His reply was something along the lines of "Of course they're not flat!". I picked one up, and sure enough, there was the slightest curve.
I've continued to look at them from time to time as they come in. Since they're shaped by hand, the curve is not always exactly the same. I've also noticed that there is more of it on the bottom end, making it not quite a "J" shape, but almost. I'm not sure if he is doing this because he thinks a slight curve on the high side is good, or if he's considering the fact that a left-hander might get one of these, and he's making a slighter allowance for that possibility. We do, after all, have side position markers on both sides, precisely for that.
As you see, there is no perfect solution. But this is one of the joys of working with Omar - finding out about wonderful old hand techniques that are often little known today. I do think a very slight "J" is a good compromise, but I can imagine what it must have been like back when fellows like Omar made everyone's guitar, and you would visit and play for them. It would have been like a buying a tailored suit - maybe even hand measurements. In the end all aspects: sound, response and hand size - cut to fit.