In MP I usually go for 3-4, just to be able to kill a scout if he attacks, total cost is ridiculously low at 6-10 gp.
As an exception with good PD nations (Jotun, Ulm) I put up to 10-11, or even more in some provinces if I'm pretty rich.
Else 11-21 is reserved for AI attack channelling in SP
Never ever I buy more than 21, it's an awful waste of gold.