im no mechanic, but...
from my experience data logging and having my car tuned.
0 knock is ideal, but you will never be able to get it like that 100% of the time.
this is due to several things i have come across.
1. bad gas is the main culprit for me. i use the same gas company and same gas and i can have knock of 3 or 5 one time and the next fill up, a 0 knock reading.
2. i have heard that our dual knock sensors are very sensitive and therefore, will register false knock at times too. either engine noise or whatever.
i will say again that this is just coming from my experience and i am in no way an authority on these things.