You are not supposed to plant seedlings until at least 2 sets of leaves have formed. Then when transplanting, you should plant the tomato up to the bottom of the 2nd set of leaves. This goes against the "norm". I always thought you planted in the same root base as in the container, but with tomatoes that is not the case. The entire stalk up to the 2nd set of leaves is burried.
We had a cold snap, so we waited an extra couple of weeks before transplanting, and we actually had a third set of leaves form. Those needed to be pinched off, disgarded in the hole (they'll just compost), and then plant up to the 2nd set of leaves. This third set of leaves will actually become additional roots. Pulling in more nutrition, and stablizing the plant as it grows taller. Here's a picture of the third set of leaves, I removed before planting.
I've planted heirloom Brandywine, Beefstake and Speckled Roman, I've had good success with seeds from Botanical Interests. I still want to plant some cherry tomatoes. I have 30 holes in the ground, and have 2-3 plants per hole. After planting, mulch was put around them, and they were watered. I'll keep you posted on their progress.