A runway heading is just that......a HEADING!
Same as a heading given during radar vectors .All Atc controllers are well aware of a reasonable upper wind velocity.Thus their projected drift angles are quite accurate.Moreover,not all aircraft are capable of flying a runway 'track'. Thus the leniency.When asked specifically to fly a track,you will be told so.ALL conventional approaches(ie ILS etc) have missed approaches based on conventional airplane ability.Therefore when the MA says fly straight ahead it means fly rwy heading.Of course it would help immensly if you couls fly the track and wouldnt be held against u,but it certainly isn't the requirment of the day.The drift caused by winds aloft(upto 3000' Agl) have been catered for and are factored into the MA procedure.
Hope this helps...