Animator MatchTarget - Call once, or repeatedly?

[limpid trail]

I've found people stating it both ways. Do you call MatchTargt once after the animation is started? Repeatedly in the time period when it would be applying? I know there's an isMatchingTarget boolean to determine whether matching is happening. But a fair amount of the code I've found calls it repeatedly, sometimes only if isMatchingTarget is false, and in some cases, only if the current animation is in the time between the start and end times.