When learning to play the piano I often emphasize to students that pianists play with their whole bodies, not just their fingers. Is it really that important to keep fingernails trimmed? The short answer is yes.
The long answer is that in order to achieve the best control of your fingers and sound, you need to be able to play on the tips of your fingers. If your nails protrude over the tops of your finger pads, then you cannot play on your finger tips. Nails are slippery and difficult to control. Finger pads have grip and friction. Long nails can also get caught between the keys and rip. Curved fingers allow you to play quickly.
How short should your nails be? Everyone’s fingers are different. When sorting out your nail length, try a T-Test. Hold up one finger and put another finger very lightly on top of it to make a T. If you can feel the nail, trim it. If you cannot feel the nail, you’re good to go!