Let's first assume a few things - because these are relevant for my decision - feel free to discuss other scenarios, but then you have to state your different assumptions
1. Gaming performance considered (for all-around use, there are already some benchmarks showing a clear benefit from lower CAS latency in some scenarios)
2. No overclocking (I don't plan to, at least initially)
I did some hasty research on this, but didn't find definite and final answers. Many years back I went with low CAS RAM (was 100 or 133MHz), because it seemed to give some 2% or so performance boost. In a $1000 system with 4GB, CL7 RAM would mean roughly a 2% increase in system price, so if the performance effect is the same, it is more than worth it. However, I've understood CPU and RAM speed are no longer significant bottlenecks in today's gaming builds, because the demands on the GPU are much higher today. Spending extra on RAM would then bring no benefit at all - just make sure you get some modern type (DDR3 1333MHz CL9) from a good brand and make sure you have enough of it.
This I suppose would be possible if you upgrade a system built today some 1-2 years down the road. Eg. you go to the latest and greatest core i9 from Intel or upgrade your 6970 to a CF configuration or otherwise get something much more powerful GPU wise. Or is the difference just so great it won't happen even just a few years down the road?
But the question in this scenario - would you expect to see about the 2% performance difference between CL9 and CL7 RAM of equal clock speed (1333MHz)? Or is this just impossible to know because we currently can't benchmark the systems 2-3 years into the future, using that day's games?
My conclusion - spending money on better RAM latency is not worth it for gamers, as long as you stick to assumption 2. Is there anything at all that speaks against it?