3-6°C between core is the most frequent. I had two Q6600. The first, was about 2°C difference only, the second, is about 5°C on load. I tried reseating many times, and also rotating my Apogee GT waterblock. In fact, as well described, I noted a trend change on the temperature difference when rotating the waterblock. It depends on the water flow through the cores, and probably on the flatness of surfaces.

This was largely debated in late 2006/early 2007 when quad cores arrived. Lapping the IHS of CPU helped most people at limiting the difference in temperature. Also, using some blocks like the Fuzion or the GTX is known to help

At 10°C difference on load, I'll definetely lap my CPU if temperature is limiting the OC