What is inportant is that the intake runners do not change in cross-sectional area throughout the entire length of the runner. This way the air can maintain a constant velocity. The runner should be slightly larger than the intake port, so that the air is accelerated through the port, resulting in better fuel mix.
I polished my intake runners to 600 grit where dry and 160 grit where wet (after the injectors). If you polish the runner too much where they are wet, you will see a symptom called fuel pooling, which is the fuel forming rivulets on the polished surface. Fuel pooling will make the engine idle very poorly.
160 grit seems to have a small problem with fuel pooling, but nothing next to 600 grit.
My engine will occasionally misfire at low fuel and air flow rates (idle, lightly loaded cruising), but the misfiring is infrequent and barely noticable.
If I were you, I would probably not go past 80 grit past the injectors if you want a good idle.
If you follow the rules of intake porting, there are major gains to be had from it.
EDIT: Here is a thread where this was discussed.
http://www.rotarycarclub.com/rotary_...ead.php?t=8550