Since it calculates nearly everything based on yesterday’s data I run it about 10 minutes after midnight, to account for time differences. The running every 15 minutes does nothing after about 1 am. My best guess as to why this happens is data and averaging being finalized by WU. Their data will vary slightly for the “yesterday” portion of the JSON for about an hour.
The only other time to run anything would be just before a set run time to make sure the wind, temp, and conditions (precipitation), are still acceptable for watering.