Many of us fertilize only in response to symptoms of needing fertilizer - most rhododendrons and azaleas are fine in soil of average fertility when the ph of the soil is correct for their needs. Acidic, or approximately 6.5 or lower.

I would not use the Miracle Grow product as directed, most would suggest you not use it at all. If organic Holly-tone by Espoma is available to you, it's a better choice if your shrubs require fertilizer.

Here is a link that might be useful: Holly-tone