A lot depends on the design and size of the boat. On my boat there are 9 lines that control the main sail (halyard, sheet, outhaul, two furling lines (actually a loop), two lines contolling the traveller position, a topping lift, and a line ajusting the vang tension. With that much other control I rarely need to adjust the traveler position. Let me explain:
I have a Hunter 33 2006. My traveler is on top of the arch that this boat has, which is between the dodger and bimini. The main sheet is attached to the end of the boom and is routed through a single pulley on the traveler, back to the boom end, then through the boom to the mast, down to the pulley on the deck, then back to the cockpit. The boom also has a vang that prevents it from rising up and the topping lift that prevents the boom from dropping. Between these two and the sail itself, the vertical position of the boom is pretty much set. So the shape of the sail is also set. In my case then, adjusting the traveler position (continuously adjustable to over about 80% of the boat width) has essentially no effect on the sail shape. It affects though the force needed to pull the main sheet. With traveler centered when sailing close to the wind and boom close to the center, the sheet pulls mostly down and very little sideways. In this situation, the distance between the boom end and the traveler is very short (about one foot or even less). I could control the boom position by moving the traveler, but that is done by hand using the two controlling lines, and in strong wind requires quite a force. What I do instead, I move traveler to the windward before pulling the main sheet (by the windlass), and get the sheet pulling sideways rather than down.
BTW, if I want a fuller sail (for lighter winds) I let the outhaul a bit. The main is not attached to the length of the boom, the bottom leech is free, so this easily controls the bulge of the sail.
