I've heard sinew referred to as natures fiberglass. It will add to the bows performance if done properly. My very first two bows were from thin ringed osage. My first one cracked and popped up a splinter. I had unknowingly violated the first ring on the back. My second bow was a stave out of the same log. I made a 75 lb bow from it that I still have. It is unbacked. That was in 92', and I was pretty much flying by the seat of my pants. Not much help back then. I have made many since then that were thin ringed and none of the osage has broken. I used to make all of my bows heavy 60-75 lb. As mother nature has caught up with me, I now make them lighter when making one for myself. They're much more of a joy to shoot for this old geezer.
If you are concerned about it, go ahead and put some sinew on it. That will give experience in that phase of bow building.