If you have experienced a problem with RenderTargeBitmap and it gave you an empty image or did not render your visual, then you must consider reading this blog post of Jaime Rodriguez.
The problem is that renderTargetBitmap is working at the Visual layer to render the visual and to do the rendering, it looks at the local properties of the visual (as expected).
Layout happens at a different (higher) layer in the platform but often ‘parents’ to your visual will apply offsets or transforms to your visual to position it, since the parent is setting these properties on your actual visual, these properties will be picked up by RenderTargetBitmap logic.