Typically, I do stick to standard ISOs. I wasn't getting the exposures i wanted with ISO200 or 400 because either the white spots get blown out or there isn't enough wheel spin. Ideally, I should've had a ND filter to help with that but they don't make one to fit my fisheye lens properly. So you just gotta make do with whatever you have.
On a crop body anything smaller than f/11 or so starts to show diffraction
and should be avoided. With that picture you could have gone to ISO100
and a wider aperture (keeping the shutter speed the same, so the motion blur
would not be changed, and blown highlights shouldn't have anything to do
with ISO, just the dynamic range and the exposure).
But that picture looks very good! I have yet to try any rolling shots.
As for partial ISO's: Yes, they are pushed/pulled from actual sensor gain
settings, but I've found that they are still incrementally better than the
higher setting. ie. ISO320 is noisier than ISO200, but it's slightly less noisy
than ISO400, so it is still advantageous to use.