No, you should be able to create your 9 patch image and use it for pretty much any size without it appearing distorted. I ran into the same problem as you trying to theme my Buttons with a StyleBoxTexture.
Double check your Margin values for the StyleBoxTexture, generally you want those to match the "corners" for your 9 patch image so the vertical and horizontal edges tile correctly.
If that doesn't help, try it with a NinePatchRect first to see if you can get it to appear correctly. The property to play with there is the Patch Margin.
I hope that helps.