Baked image is stretched I made an edge map using the pointiness node and some musgrave noise texture. 

When I try to bake this "edge map" (baking using "emit" option), however, the thin edges of the object show stretching in the baked image, unlike what the edge map looks like in rendered mode (fine dots of noise).

My screen shot shows the rendered result on the left and the baked result on the right.  Notice the stretching at the edges of the UVs.

Thanks for any help in understanding this!
The margin is intentional. It dilate the edge so that you wont background colors mixed in the edge. You can zero it out if you want to. Sometimes you will want to add dilated 
margins later if you are composing multiple bakes.