No, this is completely optional. Tsunade seldom uses Kunai. However, since she is a ninja she pretty much always has them with her. So if you want to have a perfect cosplay, you should bring 3 or 4 kunai with you.  To not get into trouble, or have quite some weight hanging on your belt, you are best off just using plastic toy kunai. You can also make these yourself out of resin or 3D print them. You will only have to paint them after you printed them or the resin has dried.