I am pretty sure I know that the answer to this question is that it doesnt matter or matter much (if it did I suspect that the manual would have said something already). But I am about to cut the tappers on my mast for the Dory (lug rig) and they are not the same on the top and bottom. So once I do this I am choosing the orientation of the mast. I also note that the scarf is also not in the middle. So that raises the question of whether anyone has found any reason to care about whether the scarf joint is on top of the mast or towards the bottom. I think that the greater stresses are likely to be on the bottom where the mast meets the thwart and where cleats are installed. But i have a feeling that the scarf joint end may being stronger in rigidity, but less flexible. on the other hand, the top of the mast will be somewhat thinner. Hence the question.
(if anyone has had an issue finding a perfectly flat level space (old house, nothing is level or straight) to glue the mast and spars scarfs together so they come out straight I found that putting down pastic sheeting on whatever surface you have under the mast, and then slidding index cards one by one under the sheeting at various spots along the mast, adding and experimenting until a long straight edge sitting on top shows no air between mast and edge along the scarf joint, works very well. once level and straight, you separate the two pieces by rolling them 90 degrees away from eachoether, add the sauce, then roll them back and clamp)
thanks