Actually, the bubbles you observe in water near the boiling point is air. There is a significant amount of dissolved air in both tap and bottled water, distilled or not. One can practically assume that water at room temperature is saturated with air. A phenomenon exists in that, contrary to solids dissolved in water, gas is more soluble at low temperatures and tends to precipitate as bubbles when heated.
Microwaved water is in no way different from the same water heated conventionally. One caution though: distilled water may tend to superheat in a microwave oven more easily than on a cook top or hot plate. Care must be taken not to heat the water too much and/or use care in handling the beaker. Superheated water will essentially explode into steam which can be painful or worse. Best practice is to heat the water slowly.
It is a good idea to heat any water used in making up developer as that gets rid of the dissolved air and that will precipitate some dissolved solids as well. My practice is to boil the water on a surface unit, quietly cool it to about 150 degrees F. and filter it. The higher temperature also helps the dissolution of the components of the developer.