I was thinking about running a few threads in two spots through the middle of the sewn end (before sewing), about shoulder-width apart from each other, then increasing the width of the hem on the ends a couple inches so the hem is triangular-shaped on each side (like the side of an hourglass, maybe.)

This might be easier than taping/clipping/pinning the seams before hemming, since you may be able to just hold the seam together with your hands as you're sewing.

Any opinions? I tried a curvy hem once, just to see how it modifies the lay but it came out all screwed up and I couldn't even get the whipping through without making an exit, lol.

Do you think a couple inches might be too much? Or is there also a better known way of doing this? I heard somebody mention that the HH models have a trick done to the whipping so it lays more flat.

On that note, is it really better, in experience, to concave the sides a bit so the ends are less floppy?