<<And what if I am closer to a terminal area, lets say on a STAR where the airway,or tolerances,are considerbly tighter, can I still sneak left or right in the same fashion without ATC approval ? ..
I suppose alot depends on the airspace you are in. Where I used to work abroad you could sneak left or right 90 degrees without any hassle as there were few aircraft. In, or approaching, a busy TMA it would be immensely dangerous to even think about such action. You should ALWAYS seek ATC approval before leaving any designated route, especially a STAR or SID, because you cannot possibly be aware of all other traffic around you.