I'd set them out the 2nd week in April.

The soil needs to be warm like when you plant caladium tubers. If the soil is wet and cold, they will tend to rot. I typically wait until the first of May but you might be able to get away with the middle of April IF it's very warm. But don't plant with these cold nights we are having now.

I grow sweet potato 2 years in a row to havest the leaves and young shoots without problem. Surprisingly, I have havested some big roots in the fall too. I planted them from the top of the plant: easy and cheap method.
I just bought one lbs of top of sweet potato plants in Korean SuperH market (it's similar to Harry Farmer Market, or any Chinese grocessry store) for about $2.00. I will wait for tomorrow when the temp is warmer, and stick those top of plants into the ground. The next week will be kind of rainy, and it's good for young shoots growing.

The trade of of this method is that I am not able to plant the variety I like... The good thing: I don't have to worry much about taking care of the seedling from the roots.

Cheers