Don't you mean outhaul rather than mainsheet in that comment ? The outhaul has a very large effect on the bottom third of the sail. Much more than the mainsheet. Also mainsheet mostly set overall twist along the leech and so is important in trimming the middle and upper thirds as well. Its twist effect on the bottom third is neglectable when compared to teh setting of the the maintraveller. In general my experience is that the bottom third of the mainsail is set with outhaul and mainsheet traveller and that both mainsheet itself and downhaul has neglectable effects here. Thus both act far more on the middle and upper parts than on the lower parts.

I do agree that the mainsail sets the "straightness" of the leech. Although prebend is directly impacting on the straightness of the middle leech. When you have too much prebend then no amount of mainsheet tension can have it stand up.

Wouter