In addition to what Arthur says about tuning for grip and form, tiller is important because most manufacturers give tuning detail for their bows, based on even tiller. None of the bows I've bought ever came with tiller being exactly even with the bolts tightened all the way down. Timing marks, ATA, brace height and draw weight are based on even tiller. Once you get everything the way the mfgr recommends and tuned to perfection, then adjusting tiller is easy, because the only adjustment you need to make after changing the tiller is with the nocking point. In fact, if your nocking point is off a hair, making very small tiller adjustments can be the easiest way to very small adjustments.

When it comes to tuning their setups, the things most overlooked by the average bowhunter - spine tuning arrows, and adjusting tiller.