It depends on a visual novel. In most cases, it makes sense to play all routes. Usually, it is possible to skip the text you've already read in the previous routes, and the new ones give a lot of new content. Generally, most plot-focused VNs are written in a way that they only give a complete story if you read every route.
In case of Tsukihime, you should definitely read everything if you want to get the complete experience.