[getsaf]

Morning dew is not enough to consider your lawn watered. Typically you want a few inches of saturated soil for watering.

As for why it's not "ideal" to run sprinklers in the middle of the day, the idea is that if you water early in the morning, then all of your sunlight hours beat on watered grass. If you just started watering in the afternoon, your grass has already been in prime morning sunlight without the watered soil to boost growing. It's not necessarily bad, it's just not ideal.