I wouldn't think so. I painted part of my GT40 intake with no sanding or other prep, and it looks fine. That being said, it probably wouldn't hurt to do this.
If you dont want to sand it smooth, I would take some 220 and sand down the whole thing with light pressure. This should help to remove any loose dirt or debris. In essence, it is more like if you just cleaned it good, but the sanding would ensure all the loose particles and dirt are removed.
Scotch Brite pads, the Gray ones, will do also, there Fine Grade...just give it a scuffing to remove the gloss if its in decent shape...I wipe em down with glass cleaner afterwards, then spray it....
Let it dry altleast over nite....otherwise you'll finger it all up at re-assy time...Been there done that more than once...