Might be good to note that when tracing buildings it is important to take parallax problems into account. Especially for tall buildings it is important to place the building footprint on the location on the ground, not simply where the roof appears in the image.
Also most buildings have corners built with a 90 degree angle. So ensuring the footprint does so also improves the quality of the data.
Imagery can be distorted, so simply painting from the imagery is not sufficient.