When you are sailing close hauled you set your sail shape using the outhaul, sheet and cunningham/halyard, and then you adjust the angle of attack using the traveler. Close hauled and close reaching the traveler is what you should be using to adjust the position of the mainsail, either for course alteration or depowering. That is the whole point and primary purpose of the traveler. Once your point of sail becomes so broad that the boom needs to go beyond the range of your traveler, you pull on the boom vang to take over the job of leech tension from the mainsheet. Then you can use the sheet to control sail angle.
EXACTLY! And if you don't have a traveler, or as in my case a very short traveler, keep the vang on and ease the main. The big thing to remember is you have SIX control lines to shape the main. Halyard, cunningham, outhaul, vang, sheet and trav. Too often folks forget the cunny and the trav.
All of these should be used before you reef. Reefing before you've gone to full outhaul and cunny is often a waste of effort and may even be slower.