I have not seen any code requirements, but I typically call for all cells to be grouted full below grade, regardless of frost zone or not. I don't like the idea of the cells potentially filling up with water.
It is not a code requirement to my knowledge either, but I have never seen structural drawings by any structural engineering company (including my own) that doesn't do that.
My thought has always been to grout all cells solid below grade. If the cells are unfilled, water will eventually fill them up and may freeze and bust the block.
The only unfilled cells I have ever seen are in house construction.
Must be a regional thing...I can't recall seeing any Engineers in my area calling for it. Grout is typically called for only in reinforced cells, which isn't often in small retail/commercial/industrial bldgs without basements. But the buildings are heated, have footings below code required frost depths, have perimeter foundation insulation, and have vapor barriers, if the water table is close to the surface.
I've also never seen a re-exposed foundation wall with the face shells blown out by freeze/thaw. (Not that it doesn't or can't happen - I just haven't seen one).
I've seen both. The CMU is usually reinforced horizontally in the bed joints with wire at 16"oc. With running bond vertical loads should disperse to appear uniform as the load reaches the bottom. So long as the wall is always in compression and the net area is sufficient for the allowable axial load, grout isn't necessary. Grout solid at least the course where the diaphrams attach or bear.
If I'm not mistaken, in California, CMU below grade foundation was required to be solid grouted. So there's minimum detailing requirements wherever you are that govern.
Typical practice in my area (Northeast US) is to grout all cells solid below grade, whether reinforced or not. Note that we always ask for grout, not concrete. They are very different materials.
