First, what piano do you have? Digital or acoustic?
If you want to record both video and audio from your phone, find a location that frames a good view of the keyboard, and that does not distort the sound. If you want to work on posture, move the camera back more to capture how you sit at the piano.
As a general guideline, avoid placing the phone directly on the piano if you are recording the audio directly from the phone, unless absolutely necessary. If you have separate mics and want more creative video angles, then you can get away with placing the phone on the piano because the mics can be placed further away.
Here are some ideas for placement:
A detailed discussion:
If you see yourself recording a lot, I would recommend investing in a portable recorder. Many options out there now. Far better audio quality than a phone, lasts longer than a phone, less expensive than a phone, and in many cases designed specifically to record music.