My Trillium does have a radius (16" according to the website), and I strongly prefer having one to not. So much so that a flat board almost feels concave to me. When I was making Otter #1, I radiused the board. My mando preference is also for a radius. But I believe it is just that... an issue of preference and comfort not right/wrong. I think that if someone was really used to a flat board, they might feel as uncomfortable with a radius as I do without.
Otter OM #1
Brian Dean OM #32
Old Wave Mandola #372
Phoenix Neoclassical #256
If you're gonna walk on thin ice, you might as well dance!