I have always wondered how often people get pulled over for inspection sticker violations. My cynical view has been that the police around here (at least in suburban Boston) do not usually pull over people for that but that is entirely my speculation. My spouse has a life-long habit of trying to squeeze an extra month out of her inspection sticker, and almost never gets them renewed on time. She has not been caught yet!
On the other hand, I remember a time many years ago when my now late wife was coming home from a horse show with her Jeep and horse trailer on a hot summer day, with one or two kids in the car and at least one horse in the trailer, and got stopped for an outdated inspection sticker. Turned out that not only was the Jeep's sticker expired but so was the sticker for the trailer (and yes, they need to be inspected, as well!). The cop also took pity on her (what with the kids and all) and gave her a warning. That might have even been long enough ago that Mass. still had the separate police in the Registry of Motor Vehicles, and I suspect they were more attentive to things like inspection stickers because that was their specific scope.
So, I guess it is the luck of the draw -- and I personally try not to miss my renewal 9although i have been a day or two late once or twice).