On and off points are enough, but feel free to add in and out points

On and off points belong to specific partitions, so they also serve as concrete test cases for the partitions. This means testing all the boundaries of your input domain is enough. Nevertheless, I often try some in and out points in my tests. They are redundant, because the on and off points exercise the same partition as the in and out points; but these extra points give me a better understanding of the program and may better represent real-life inputs. Striving for the leanest test suite is always a good idea, but a few extra points are fine.