It is not necessary to cook rhubarb before making crumble, but you can if you like. If you do cook the rhubarb, it will be softer and more tender, releasing some of the tartness rhubarb is known for. Precooking can be a good option if you are using rhubarb that is not very ripe. To cook rhubarb, simply cut it into pieces and simmer it in a saucepan with a little water or juice until it is soft. If you do not cook the rhubarb, it will retain its tartness and more of its crunchiness. This can be a good option if you like a more tart crumble. Ultimately, the decision of whether or not to cook the rhubarb before making a crumble is up to you. There is no right or wrong answer, it simply comes down to personal preference.