Dual CPU Memory Configurations

I Have a Precision T5500 with the following DIMM's populated with Single Rank modules.

MB DIMM1 - 1600MHZ - 4GB

MB DIMM2 - 1600MHZ - 4GB

MB DIMM3 - 1600MHZ - 4GB

Raiser DIMM1 - 1600MHZ - 4GB

Raiser DIMM2 - 1600MHZ - 4GB

Raiser DIMM3 - 1600MHZ - 4GB

Looking at the systems memory configuration I should be using Dual Rank, what is the repercussion using the Single Rank modules? I have had no BSOD's

Should I get Dual Ranks modules?

Thanks