Preheat oven to 350 degrees F. Lightly grease a 15x10x1-inch baking pan. In a large skillet cook bacon until almost crisp. Using a slotted spoon, remove bacon from skillet; discard fat. Drain bacon on paper towels.
Clean mushrooms; remove and discard stems.
In a medium bowl stir together cream cheese and cheddar cheese. Stir in reserved bacon and, if you like, garlic. Spoon filling into mushroom tops, and place on a greased 15x10x1-inch baking pan.
Bake for 25 to 30 minutes or until mushrooms are tender. Let stand 5 minutes before serving.