Just to be safe, On your last month of non-reporting, stay clean. I've seen several "non-reporting" friends get a drug test request just before they get off probation. Most of them got screwed.
On the otherhand, if your PO is cool like mine was, They my not care about herb at all. My PO and I sat down after my repeated failures, He just cut straight with me, He offered to give me 30days in exchange for my 1-1/2 yrs of probation left(knowing I wasn't gonna quit). And the charge was still dismissed(which wasn't herb related).
I say play it safe, at least at the end.
They catch you when your guards down.