You don’t have to make them fill the whole grid – it’s just that the more space they cover the higher the resolution of the texture will be as you give the model more pixels to work with.
You can scale and rotate the UVs just like you can with geometry to fit the grid better and then PIN them so when you repack the other UV islands they won’t be affected.
You don’t have to move the islands before averaging and packing again. It just helps to see what you’re working on and makes selection easier.
