If you use the tab on the washer as a point to pull the cable against, then you will not pull it up tight against the threads of the anchor screw. If you don't pull it up aginst the screw threads, then you will not ding the screw threads or cable when tightening the screw.
Cable position, inside or outside of the screw does make a small difference in the indexing of the derailleur. Its enough to make the difference between so/so and perfect shifting.
Generally, you will find that the major manufacturers have spent serious time designing their equipment and that following their diagrams or instructions will provide pretty good results.
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