Talking generally - depends on the type of approach being flown - ILS/MLS approaches (called precision approaches) are normally aligned with the runway, but can be offset up to 5 deg. Non-precision approaches (VOR/NDB) can be offset by up to 15 deg for Cat C/D aircraft (B737 and larger). An aircraft established on a published IFR approach procedure should not be asked to alter heading by ATC, unless there is a safety of flight issue.