It depends. I assume there is a budget for this build. I am also making the assumption that you are purposely opting to leave two memory slot open and unused. The better question I think you should be asking, pardon my audacity, is whether it is a better bang for your buck. There are several components in a system that affect different functions of the performance of the overall system. System memory seems at current prices to offer diminishing returns if you could invest the bare required minimum for system memory and devote the difference into a better graphics cards for gaming, better and faster storage, or even liquid cooling for higher over-clocking. In terms of performance/price, system memory is the highest fruit, and I think you should always optimize other components for improvements and leave system memory for last, given the current market situation.
Unless you are going for the best of the best, and MONEY IS NO OBJECT, in fact, it is burning a hole in your pocket, and you are financially secure to spend it, then even then it doesn't make sense to make this decision because it would be better to buy four modules at the highest overclocked speed, e.g- 4000 Mhz. You must be absolutely certain that you are opening a can of worms. Because overclocking memory is an even trickier business than a CPU. It is not as rosy as you are led to believe. You seem to be penny wise, pound foolish if you are willing to spend so much money on this just to save on shipping when a bigger delta of performance per price could be achieved elsewhere.
My two cents. We wouldn't be doing right by you if we didn't advise you to get the best bang for your buck.