It's tricky. On the one hand, they tell you to keep a steady beat. On the other, they tell you that some types of piano music require a more fluid approach. Sometimes you need to add rubato to your playing, so the music ebbs and flows, relaxing in some places and pushing forward in others. This can be especially useful in Romantic piano music by composers such as Chopin and Schubert. But it won't get you very far if you play in a rock band.