Recording piano is my speciality.
Taking the diagram below:-

Position "B" would be my first choice. position "C" will give you a lot of pedal noise and position "A" will give you a lot of hammer noise.
I normally use a pair of microphones and almost always omnis as a directional mic. misses the bottom end (though not so important with an upright).
You also need to be careful of where the piano is in the room to avoid the "honk" caused by standing waves and problems caused by reflections from the walls, etc.
In the first instance, get someone else to play the piano and go and listen to how it sounds in various positions - where it sounds best to your ears is almost certainly the best place to put the microphone.
I hope this helps.
John