At least for body shots, I always get punished if I accidentally don't tense up, or realize too late that I have to tense up. Also, if you're 'going into' an attack to sort of jam it, then I'd tense up too. For shots to the head, I don't try to be overly tense but I make sure it's not too loose, so in the middle. For my legs, I just aim to be firm, but not tense at all so I can possibly squeeze in a counter attack.

Course, I'm no expert. X_x;;