According to the Daily Pennsylvanian, 74% of all law schools have decided to evaluate law school applicants based on their highest score. This change comes on the heels of an ABA policy change asking schools to report their matriculants’ highest LSAT score as opposed to the average score used previously by 87% of all law schools. That’s quite a switch.
While the change will mitigate the impact of the proverbial "bad day," schools still warn against trying to game the system by taking the LSAT endlessly until you get a high score.
My recommendation: Prepare for it and prepare well the first time. Take lots of practice exams. Aim to take it only once. If you have that bad day, then you have the option of a retake without the outcome being dragged down by the first score and without concern that an even worse day will further damage your admissions chances.