Steps to reproduce:
1. Download and open attached project "RenderViewportScaleBug.zip"
2. Make sure VR mode is on
3. Play scene "scene"
4. On Main Camera object, "VR Render Settings", lower "Render Viewport Scale"

Actual result: Small jittering can be seen around the edges, even if the HMD is kept completely still (See: https://gyazo.com/e831800c96ff54b6cef274b8e9b36869)
Expected: No jittering

Tested and reproduced on:
Windows 10
Unity 5.4.0f2

Could not test on 5.3 or below, because the option did not exist
Neither on 5.5, other issue persists