What is the difference between R&R percent of tolerance and percent of process?  When to you use each one?

Both are used to express different aspects of a product, one to tell if it’s good or bad and evaluate the measurement system and the other to show the usefulness of a product, so you use one or another according to what you are trying to evaluate and express. Percent of tolerance (also called precision-to-tolerance ratio) is the percentage of the tolerance taken up by the measurement error, and so it helps classify parts as good or bad. In other words, it measures variation in the measurement from the device and whoever is taking the measurement, and indicates the amount of variation that is permitted between all parts produced. Percent of process indicates the relative usefulness of the gage system for use in control charting and process capability analysis.

What is guard banding and do you think it is a good practice?

Guard banding is a very important tool that determines the pass-fail risk of a product. Shows the acceptance rate of a product by setting test limits lower than Specification limits (SL) in order to optimize False-Accept & False-Reject risks. It’s a very risky tool to use, but I think it is a good tool if you are sure of your data, because you can find out the acceptance of a product and increase the confidence that a device is really in-specification by factoring in the measurement errors.