I usually use 216 on them. Remember to cut the sheet large enough that you can open the barndoors a bit and wrap the gel around the outside. Otherwise there isn't much point -- the softness of the light is dependent on the size of the source relative to the distance to the subject, and putting diffusion right over the tubes (though tempting because you can still use the barndoors and eggcrate) hardly does any softening at all because you haven't created a larger surface area for the diffused light.
I use black showcard barndoor extenders held up with paper clamps to extend the barndoors when I use diffusion over the unit.
You don't end up softening a lot, but it does take some of that Kino "shine" down plus it spreads the light better when you start switching off individual tubes. For example, if you ended up only using one tube in a 4-bank unit, the diffusion over the doors helps keep the light from getting sharper.