You will be just fine using miracle grow to foliar feed your azaleas.
I use a similar water soluble fertilizer in the nursery plus iron and
manganese chelate with every pesticide application and the plants are
fine. Make sure you follow the label rates, because too much applied
to the leaves can definitley cause damage, especially if it is a hot
day. Maybe use half the label rate and go from there. for what its
worth, applying the fertilizer directly to the root zone will be more
effecient and useful, but the foiliar feed is good for plants that are
visibly deficient in iron, manganese, or nitrogen. The foliar route
also dosent last very long, but works quicker. As for the bit about
water acting as a magnifying glass, thats a bunch of voodoo mostly.
The water droplet would have to be so thick that the leaf probably
couldnt support it anyway, and if it was a sunny enough day for the
magnifying effect to take place, the water would evaporate before it
even had a chance. Another point- when you use a magnifying glass to
burn something, what happens if you hold it right up against the
object?? Nothing, because the beam has to be focused, and in this
case, it wouldnt be. Simply put, its a myth.