The peening hammer is used on the blade only when the sharpening has worked back on the width of the blade, at that point the blade gets thicker, by penning all you are doing is thinning the blade again.
Just remember sharpening a scythe blade is not like sharpening a knife blade, its putting a sharp edge which under close looking is a saw edge, and just remember its only the first 3 inches that cuts, the curve and the length of the blade is for removing the cut grass away from the next cut.
I for one use scythes every day in my working life