Water as often as needed, with or without Miracle grow. Just don't spray the plant with water.
I had the same problem years ago and an old gardener told me what to do.
If you spray the plant in the evening, the water will collect at the bottom of the fruit and stay there all night causing it to rot. If you water in the morning the water will evaperate and not collect on the tomatoes. I used to go out in the evening and really spray everything. Now I water in the morning and just the base.
Seems to be worse in the dry years when everyone is watering alot.
Try it, may not stop all the rot, but it will be alot better.