Use negative points only if you have good reasons to. Many authors don't recommend normalization or negative points for the following reasons:

 Correction methods include student's personality into the evaluation process. Some students are reluctant to gamble, while others don't hesitate to. Introduction of negative points system reflects more on female students which are usually unwilling to take the risk.
 There is no reliable data that corrected points relate better to students' knowledge, so additional calculations are unnecessary and useless.
 Students are similarly ranked with corrected and uncorrected answers, so the choice of minimum required score is more important than correction.
 It often happens that, because of lack of time, students really guess the questions at the end of the test randomly, which excludes the advantage of negative points for partial knowledge.

 It is always badly accepted among students as an unfair punishment, so consider carefully whether you really need it.
 

 Almost the same results can be achieved by correctly calculating the minimum required score (by raising it, to be more precise)!