You may cancel IFR with ATC at any time in VFR conditions below 18,000 feet.
You may cancel with ATC or FSS upon completing a flight (on the ground).
Do not cancel in the air while on approach to a surface-based Class E airport unless the reported weather meets the basic VFR weather requirements of 91.155: visibility — 3 sm; cloud clearance — 500 feet below, 1,000 feet above, 2,000 feet horizontally. 91.155(c): Except as provided in §91.157 [Special VFR], no person may operate an aircraft beneath the ceiling under VFR within the lateral boundaries of controlled airspace designated to the surface for an airport when the ceiling is less than 1,000 feet.
Do not cancel in the air while on approach to an airport in Class B, C, D or E airspace if that airport does not meet the basic VFR weather minimums and cloud separation requirements of 91.155 (unless you receive a Special VFR clearance). Of course, canceling on approach to an airport with an operating control tower is quite simply a senseless (not to mention bizarre) waste of time. Note: A Special VFR clearance is treated almost the same as an IFR clearance as far as separation is concerned. It is not likely to save you or the guy behind you any time. If the reported ceiling is below 1,000 feet or visibility below 3 sm, just wait till you're on the ground to cancel. You never know who might be lurking in the weeds just waiting for a chance to enforce the rules and make your life miserable.
If arriving at an airport with an operating control tower, the flight plan is automatically closed upon landing.
If you fail to cancel your IFR (or VFR) flight plan within 30 minutes of your ETA, search and rescue procedures will be initiated. The sheriff will come looking for you. (Verified from personal experience.)
Additional VFR requirements of OpSpec C077 for Part 135 turbojets and Part 121 aircraft include: