I recently did my first sinew job with bovine sinew, and some of the strips still had a bit of sheath left on them. The bits with the sheath do not lay super well with the other strips of soaked sinew, as they arnt as flexible, so its tough to get a good flat sinew job. However, I used the PatM sinew technique, wihich requires wrapping and reheating, and that brought it all together into a nice smooth matrix. Next time, I will be diligent about removing more of the sheath, but I don't think the performance of the finished product was harmed.