If I'm not getting in over my head, I believe that silver-halide crystal formation is governed by the pAg of the solution in which it is formed, in addition to the soluble salts & other things (silver-halide solvents, etc.) present.

According to the famous graph that's posted hither and thither, there is no square grain stage. And like Terry says, they'd be so small that you wouldn't really see them as individual squares anyway.