I am just wondering what the convention is for putting designators for tightly packed areas. It is best just to put arrows to the designators, make the designator smaller and put it inside the component etc Here is an example, the R4-R12 are all the resisters below IC1.
Around 0.15 min line width and 1.5 mm height should be big enough.
There isn't any standard about how to arrange the ref des in the PCB. Just try to make it as useful as you can. That's the reason that there is no point in putting it underneath of components.
As some mentioned, ellipsis (Rx.. Ry) are fine, as well rearranging the ref des in the same way than the physical components in a space nearby.
The key point is thinking about who is going to read/use that silkscreen.
If something is ambiguous, I would delete it. It can cause more problems.