Should I resurface my concrete before laying epoxy on a garage floor?
With the epoxies I have used minor imperfections like small divots from a heavy object chipping the floor are usually filled in. The epoxy can make a very slick surface. For instance, I coated one bay of my shop that I do automotive work in and wanted it like glass so oil spills were easier to clean. This worked great until I spilled some antifreeze. The antifreeze was so slick I could not stand on it until rinsed several times. I ended going back and added another coat with some traction powder. It is still smooth and easy to clean but not as slippery.  

The epoxy did fill minor divots where heavy items had chipped the surface. 

As a note of caution make sure to get epoxy specifically made for garage flooring. I always use 2 part and have excellent long term results. A friend just used a basic epoxy paint not specifically made for garages and when he brought his truck in and parked it the heat from the tires caused the epoxy to fail under the tires he had spots all around where he parked that the epoxy peeled off. He ended up having the floor shot blasted to remove the epoxy and recoated with a 2 part garage epoxy then had no further problems.