I also bubble in groups, right before I turn the page, and have always done it that way. However, at the 5 five minute warning time, if I am not within the last couple of questions, then I change it up to make sure I get my guesses in on the questions that I won't have time to get to. I will go to the bubbles for the last question and bubble all D's from there backwards. Fill them fully, but lightly so that they can be erased if you get back to that question. Then I go forwards again and bubble just one question at a time.