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The grid settings depend entirely on what components there are on the board. If I'm routing traces between pins on a big IC, the snap and electrical grids are set to center traces between pads. If I'm doing a lot of closely spaced work, I might set the grids down to 1mil, or even tenths of a mil to give me more control over the specific area I'm working on.
The component grid depends on the density of the board and whether the device datasheets are in metric or imperial units. A typical medium to low density board might use 20mil or 25mil component grid.
Which units you use are up to you. If I'm working with components that use MM (metric) units on the datasheets, I usually toggle to metric display. If the datasheets use mils (imperial) units, I toggle to imperial display.
Most of my work is done in MILS, and I'm most comfortable thinking in Imperial units. You should use whatever works best for you.