I had friends that got busted for coke and crack and heroin. Their PO was like "well...if you take piss tests and none of that bad shit shows up, its better than nothing" and she'd just turn a blind eye to weed.
Then again, the friends that got busted for weed didn't get the same leniency. I guess for some PO's, they'll make a personal decision based on how bad you are, the severity of what you did, whether you're in trouble for a drug offense, etc.
So i'd say its on a case by case/ PO by PO basis.
Do you have regular meetings, or do you get random phone calls with numbers? A lot of heads i knew got random colors or numbers (red, blue, green, black) and they'd announce a color (Black) and everyone with that color had to go in for a piss test. It kept people on their toes. Some colors were more susceptible to piss tests than others (i.e. Brown went in 4 times a month, red went in the first week of the month, blue went in the second week of the month, black went in the 3rd week, white went in twice a month, etc.)