The best analogy I have found: When you shave, do you plunge the razor into your face, or do you follow the contour? Well, when you cut plastic, shave it. When you cut wood, it really will not penalize you for plunging. Plastic will break.
Luke, Ed described it perfectly. There are many variations of blanks that fit into the plastics category; some are very brittle while others are more forgiving. Just take your time & don't get discouraged if one "chips out."
Also, if it's a translucent blank you may need to paint the tube & the inside of the blank.