I would go for 'normal'.
A gazillion people have pianos over the centuries, most in rooms with less humidity, the thing is to avoid excessive dryness that shrinks moldings, cracks wood, etc. In 'normal' humidity levels, they don't crack, they hold their tune.
And definitely avoid frequent temperature changes, i.e. avoid outside walls, too near windows, avoid too much sunlight on the wood ...
I have a forced air system with adjustable humidifier. I watch my ceiling moldings in the winter, and how much moisture jumps on the windows and storm doors when I open the outside doors. Enough, not too much.
