Watering in the heat of the day can waste water through evaporation, but the cooling effect should benefit the grass. There is no truth to the idea that water droplets act as magnifying lenses, causing burn. Watering late in the day is efficient but some custodians have an aversion to leaving a lawn "wet" through the night for fear of encouraging disease (much of the time, dew wets the grass anyway). This leaves early morning as perhaps very slightly favored for watering the lawn.