I have found that if I place 6 small pieces of 3/4 pine in a planting tray with no holes and sit the tray my Wheatgrass is growing in, on top of the pine blocks I have the ideal setup for watering from below. I let all the water get used up by the plants before I add about 12 ounces of water to the bottom tray. This amount of water does not reach over the blocks so the roots must stretch out to reach the water. I don't believe it is good to put so much water in the bottom that the roots are in the water all the time. This method has just about eliminated my mold problem.
As an aside I have found that the large single compartment plastic containers used at salad bars in grocery stores works great for storing cut wheatgrass.