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With arithmetic I think it is the only way for sure if automatic recall is desired. Ditto with declension of verbs. The desired state is not 'mastery' as much as 'reflex'.
Where I have the problem is with teachers insisting on 'showing work' for self evident facts in arithmetic. All I cared about with my DD was whether or not the answer to an arithmetic problem was correct, at first. Later, I wanted her to understand different strategies for simplifying calculations but that would not have been possible without an underlying mastery of the 'rules of arithmetic' to start with.
