Adding a border to your render gives you a better chance to eliminate white edges that may appear.
So get your 'Paths' Tool, and being rendering.
Once you're done rendering, go to Layers>Transparency>Add Alpha Channel
That should make it so when you cut, you will have just the image that you want.
In the 'Paths' Tool Option Window, you have to click on "Create Selection From Path." That should trace the render.
Then you click CTRL + I to invert the cutting of the image that you want, to the outer part. (The surroundings of the render.)
But before you invert it, go to Tools>Paths. You should see a kind of feather on top of a pencil.
Click it, and you will have the border set in.
After that, then you invert it.
Click CTRL + X to cut it.
Then you have an outstanding render!
Please do Not rip.