Judging by their cold-hardiness, I think the seed could be sewn directly into the garden with other early seeds, but I think I would start them inside and transplant them to their beds just so I could control number and location. I've transplanted crowns at all different times; fall seems to be best but they've grown even when transplanted in summer.

I've also read that rhubarb doesn't grow true from seed, so you may have some issues there.

Ray