During exam weeks, Slytherins charm the common room walls to say: Better Done Than Perfect. Slytherins have perfectionist tendencies and stress tends to create analysis paralysis.
The house head and prefects are not worried that the quote will lower the quality of work Slytherins turn in.
Slytherins care too much about the quality of their work to turn in something shitty. This quote helps Slytherins shake off paralysis and actually start and finish their work.