Honestly it depends on the specific workout and the type of intervals. Some intervals, such as shorter on/off VO2Max intervals you're not meant to fully recover and so taking extra rest would be 'cheating'. Better in that case to drop the intensity a little. On the other hand 3-5min max intervals a little extra rest might be OK.

I've also found that some TR workouts, the rest interval may be longer or shorter than ideal simply because they'retrying to fit a specific amount of work into a specific duration workout.