Any tomato seedlings that have developed their first real set of leaves should be repotted into bigger containers to keep them happy. Mine already had roots sticking out the bottom of the cell packs. Choose the strongest seedlings, and plant them deeper in their new pots. Repot them at least one more time before they are ready to go outside, planting them deeper again. This will minimize them getting too leggy from growing indoors, and the buried stem will send out new roots, helping the plant become stronger.
Don't you love the smell of tomato leaves? Having tomato seedlings in the basement makes me feel like I am gardening already!