some vanes in the middle of the disc are directional to work better like AP and Brembos - imagine your disc as a sandwitch of the 2 sides - I'm taking about the vanes in the middle - cheaper discs - these are usually just straight so it does not matter but on something like an AP they are angled to work better and draw air through the disc more cooling them better. If you put these round the wrong way you are making it much harder for the air to be drawn through. See what yours are before worrying about the grooves - they are probably straight though but check.