The answer to this depends entirely on where you are.. .. .If you're on an airway in one of the more distant parts of the world - NAT tracks, Africa, Siberia, Northern Canada etc etc etc - then IMHO it makes sense to turn off the airway (giving due thought to high ground etc) as soon as you can. The most likely place for conflicitng traffic to be is on the same airway as you are. As for contacting ATC first, well in some of those places........ .. .If you're in congested airspace - Europe, New York TMA, LAX TMA etc etc - then you've probably got an equal chance of hitting other traffic whatever you do - turn, or continue straight. The safest thing then, if you've got the time, is to talk to ATC before you start down (but the emphasis has to be "if you've got time"!!). .. .It's horses for courses.....