All the SIDs in the region I'm flying in start with 'Climb straight ahead......'. I think that's fairly self explanatory. Having said that though, very few do. I onced asked a contoller at a meeting with ATC this very question. His answer was that if given by the tower they are looking for runway track, if given after departure then it will be a heading. Made perfect sense to me but unless it's in writing then we're going to end back on this forum....