I have found there's no problem watering the entire tree provided there's sufficient air movement to prevent fungal growth. For me this means I can water everything all over until about May, when the breeze stops blowing and the humidity rises to a thousand percent or so. It's important to bear in mind that when we water a two foot tall tree that's sitting a few feet off the ground, there's a big challenge in getting enough air movement to penetrate the foliar mass of the tree. Specimens that are 20 or 30 or 80 feet tall don't have this same problem, even in the depths of summer with no air movement. The interiors of these trees remain open enough to allow for excess moisture from the rains to dissipate. Not so much with our small trees. I try to adjust my watering practice as the season progresses to prevent fungal growth, and that means watering only the soil to the greatest extent possible (not always possible when I have to use my automatic overhead watering system).