Where should I sit with respect to the keyboard? Should I sit directly in front of the middle C, or is it ok if I shift slightly to the right. I noticed that pieces requiring my left hand to work close to the middle C are easier to play if I sit to the right of the center.
Or is this bad practice, and I should always sit directly in front of the center?

My impression is most people say sit right in front of middle D, not middle C.

I use middle D, or even slightly higher, as my center.

Mel

dannylux is right. if you take close look to the middle D, you'd see the entire keyboard is mirrored in left and right from that D, which makes sense to let it be center of keyboard. however, sitting position might also depend on pieces you playing. some piece may have a lot of bass notes at far end of register and you'd prefer to move yourself to little left; and on the other hand, you want to move to little right playing a piece with a lot treble notes at high end. it's all relative.