When a RenderTexture is created through the editor, the editor restricts the size to a power of two.
In my case I want the RenderTexture size to match the screen resolution which usually is not a power of two, so the power of two restriction means I have to either over or under sample when I render the RenderTexture content to screen.
While looking into this, I noticed that if I create a RenderTexture through script I can specify any size I want and everything seems to work fine.
My question - is there a reason for the editor restriction?
It depends entirely on the platform. My guess is that some platforms require power-of-two render textures, but most mobile, desktop, and console GPUs that I know of no longer require PoT Render Targets. In the end, if it works for you, ship it! 