We have been told that runway hdg is just that-it might not be straight line out from the runway in a crosswind. If we want an aircraft to fly the extended centerline then our instruction must be 'track extended runway centerline' followed by the rest of the clearance/climbout instructions.
Wearing my other hat i have noticed that when paralleling aircraft x number of miles apart both can report hdg's up to 15+ degress apart even tho both are tracking parallel. When both are asked to confirm magnetic heading one is invariably in trk mode.
Over and out