Hmmm, good question. Well, one things for sure, if you have directional tires, you only swap front for rear on the same size.
Honestly, this is typically what I do as well (same side front to rear), even with my non-directional Nittos. The logic behind this was shared with me by a mechanic a while ago: The reversal of rotation can fatigue the belts in the tires, since they are spinning and loading in the opposite direction that they were spinning before. I don't know if it applies to the current crop of tires, but I still do it.
Matt