Most of the texture makers are used to the distortion, and they know where to draw things and how to make them to look good on the head.
OR
They just draw on a 3d sphere in photoshop so they don't have to deal with that.
Drawing on the sphere and using those lines as guidelines is what I used to do.
Some others like to make the canvas width twice as wide.
Drawing on the sphere gives the highest quality, though.