people make the mistake of assuming that everyone will DI "correctly" all of the time, but this is never true. in reality, even the top level pros will DI well, but some combos can be performed no matter which way you DI, and its up to the person doing the combo to react quick enough to take advantage. theres no such thing as "perfect DI", its merely dependent on what your opponent attacks with.
for example, if i dair a fox at mid %, they will pop up right by my head or so, open for a free hit. now, in most cases i would just knee. if the fox is expecting my knee, he will hold up, DIing so the knee wont kill him or knock him far off the stage. if your opponent gets in that groove of DIing all of your knees up, then insert nairs and uairs where those knees were. he will merely be caught in the combo, popping around until you have an easy knee with more % and better stage position. i often get frustrated with people who get caught in one of my nasty combos, and will then say "man, I di'ed perfectly for you". while that may be partially true, they think that they are reacting incorrectly to what I am doing, but really, im just reacting correctly to what THEY are doing.