Truing rear wheel with gears cassette I have a 700cc rear wheel with gear cassette. The centering or dish of the wheel appears to balance towards the gear cassette. This means when i use a tool like the below image, that the hub (and the hub lock nuts) is not in the centre. Should I be truing the rear wheel to be centered on the dish or is it off-set towards the gear cassette for a reason?
You rim should be centered over your hub. If you are using the tool correctly, and the rim is not centered, it should be redished. 

If you are not confident in your abilities and knowledge on the subject, get a second opinion from your LBS mechanic before you make changes to the wheel. Sometimes, those kind of changes are difficult to reverse.