It isn't just how often you water your trees, but how you water your trees. For trees, I like to set a hose on top of the root ball with the water turned on to a slow trickle- it shouldn't run off into your lawn or beds. Depending on the size of the tree, maybe 20-30 minutes should suffice, and you may want to move the hose to the opposite side of the root ball half way through the watering. Before you water again, get down on your hands and knees, pull the mulch back, and check to see if the root ball is moist. If its dry, water again. If it is moist, leave it alone. I have seen many trees over watered, and they may initially display the same signs of stress as being dry. Over watering can kill a plant.
If a plant is really dry, water may just run off the root ball without soaking in. If this is the case, spray the root ball several times to get it to accept water.
Most trees will do just fine planted close together. They will form a mass or hedge, and where they touch and grow together the foliage will die out, but the rest of the tree should be fine.