I have only made a couple of sinew backed osage bows, I didn't degrease just roughed up the back with a toothing plane.
Dean Torges was of the opinion that freshly worked osage didn't need to be degreased. I found this to be true as well. I once degreased the backs of all my BBOs before I applied bamboo. I quit degreasing after about the 20th one, I didn't have any failures on the next 30 or so.