Moisture levels are critical to the healthy development of seedlings.
Keep seeds at 100% moisture until they germinate. But once they pop through the soil, don't apply more water until the cell begins to dry out.
Keeping the substrate wet after germination can encourage root rot, but keeping it too dry during the germination phase can kill the seed.
Finding the balance is critical to the process, but fortunately, it's not difficult to do.
Constantly wetting the soil while seeds are in germination is ok, because the seeds need to absorb moisture to trigger the germination process. Unlike germinating seeds in a cup of water, saturated substrate still contains many oxygen pockets, so there is no risk of the seed rotting.
However, once the seedling penetrates the soil, letting the substrate dry down will encourage healthy root development.
A safe rule of thumb is not to irrigate the cell until you see the tap root sticking through the bottom of the seedling tray.