[Agda] when to set underscore suffix?

[mechvel at scico.botik.ru]

People,

Standard library has the  ¬_  function for negation.
Probably, the underscore is set for a better parsing when  ¬  is 
composed with
something (?).
But for example,
         length : List A -> Nat

is not suffixed with underscore.
What is the difference?
When underscore is good in an unary function name?

Thanks,

------
Sergei