New construction or repair?
In general, it is easier to remove material from a "positive" shape than a "negative" shape. For example, it is physically easier to remove material from a tenon rather than its mating mortise. Similarly, it is often easier to remove material from the positive "tail" than the negative socket.
But, it depends upon the specific circumstances. If, for example, you have a "bump" protruding on the interior of your mortise, the best fit occurs when you remove the bump. Some of it depends upon assessing the geometry you have and taking the appropriate steps to have the two mate. Sometimes that is the positive shape, sometimes the negative.
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