Is it important to consider the cable resistance when using strain gauges?

The cable resistance can be a significant factor in the accuracy of the overall measurement. It is very important indeed to consider this point.

Prosig software allows the cable resistance to be entered by the user. Once this resistance has been entered, any calculations or calibrations carried out will be based on the known values of the bridge resistors as well as the resistances of all of the cables.

In most cases this will be negligible except if long cables are used.

When should I use the ground connection on my P8000 data acquisition system?

The ground connection should be used if an issue has been observed in measured data, such as significant electricity supply (50Hz or 60Hz) distortion or significant noise.

It must be attached it to a very good earth point (like a steel pipe or foundation)

It should be noted that sometimes connecting the ground cable to earth can actually make matters worse. It depends on the situation.