Both
The sheet is eased to set the sail to the wind.The traveler on the headsail is eased so that the leech may be drawn down while the sail itself is allowed to be set for the wind. Otherwise easing the sheet would allow the top of headsail to twist too much dumping valuable air.On the main, the twist can be controlled by the vang so moving the traveler isn't as critical, unless there is not vang, then the same principle would hold.