Step Size - This value is a percentage of voxel size. 75% is good for most uses; for transparent smoke you may use up to 300%. Less than 25% should never be needed, except when attempting an effective use of Jittering.
Jittering - This value is a percentage of voxel size. It can help to reduce possible artifacts and/or soften shadows. Transparency for a volume of smoke is not taken precisely along the ray that is being traced, but rather, with a random offset whose maximum is set by this parameter. It has to be combined with small Step Size (5% - 50%) to be effective.