As soon as the seedling shoots emerge and stand up straight, they prefer cooler temperatures, so you can remove artificial heat and concentrate on putting the flats in the best available light conditions. This fact also helps you economize on buying heat mats or cables. Since you start different types of seeds at different times, the early ones are through with their bottom heat in time for the next batch. It doesn't matter if it's leaky. Water Moisture is required for seed germination.
Most seeds remain dormant until they are in a warm environment where they can soak up water.
It's also best to avoid a sudden gush of water that could displace seeds or knock over tiny seedlings. One way of doing this is to use a fine-nozzle sprayer to gently mist seedlings. Bottom watering works even better. This is where using peat pots in a plastic tray comes in handy. Pour water into the tray around the pots and let it wick up, moistening the medium indirectly.