When routing near controlled airspace (CAS), GASCo recommends that pilots plan to remain clear of the horizontal and vertical boundaries of the airspace by a suitable distance that's appropriate for them, their aircraft and the prevailing conditions. As a general rule of thumb 'Take Two' (i.e. 2 nautical miles horizontally and 200 feet vertically) would seem to be sound practical advice but in some cases it might be prudent to allow even more. After all, it only takes a small distraction, a moment's inattention or a bit of turbulence in the atmosphere to gain a hundred feet or more.
If pilots need to fly close to the boundaries of CAS then it is strongly recommended that they contact the ATC unit that is controlling the airspace and announce their intentions. In so doing their aircraft becomes 'known' traffic thereby removing uncertainty about its subsequent flight path from the controller's mind.
Consider this example, an airliner is descending and is being vectored to the ILS via a radar base leg when the controller observes a return from an aircraft operating close to the zone boundary which is not in radio contact and which if it continues on its present heading, will enter CAS and come into conflict with the airliner. The controller has only seconds to decide what to do. Does the controller stop the airliner's descent thereby disrupting it and other aircraft in the process to be certain that separation will not be lost or does the controller deem that the unknown aircraft will remain outside CAS and let the airliner carry on? So much easier and safer for everyone if pilots Take Two and wear a Listening Squawk when flying in the vicinity of CAS which in most parts of the country is likely to be the case, calling Air Traffic if they need to operate close to the boundaries for any reason.