I think on the Youtube videos they use pre-cuts to demonstrate that you can use the Go! without a lot of waste, similar to cutting with rulers. Also, it is just neat looking.
When I first got my Go! (mine is manual crank), I was very careful to cut pieces similar to the demo's, to make as little waste as possible. However, as I have gotten used to it, I no longer do nearly as much pre-cutting and accept that a little fabric waste is worth my time and energy. I think I have used fabric cuts as large as a yard; certainly half a yard works well. That is all the pre-cutting I do.
My basic method is to fold the yardage over so I have two doubled layers of fabric on the cutting template (four layers of fabric). Usually I want to cut all of the pieces on the template; however, if I want to cut only one, I carefully position the mat to cover only that one piece. Then I crank the whole thing through. It's fine to have excess fabric going under the roller. Usually I have the excess fabric positioned to one side, but you can have it spilling over on both sides of the die if you like; it's just more wasteful. It's also fine to have excess fabric running through behind the dye. If you experiment with a one-yard cut of yardage (or half a yard if one yard is too much for you), you will discover how all this works.
Actually, if I think about it, I think I get less waste by using the larger pre-cuts. If I get any more waste at all, it is very little. This is because you can position the larger fabric pieces just as you want, close to the last cuts made in the fabric.
Hope this is a little more clear than mud......