My experience is that 4 sticks installed in four adjacent slots will get fairly warm if you choose to run the XMP profile or are overclocking things in other ways. I have also, albeit infrequently, had RAM that would start to throw errors running stress tests when the memory gets warm.
My conclusion is that if I can't hold my hand on the RAM comfortably while running memtest86 or the Prime95 memory stress test, I consider adding a cooling fan. It takes very little air flow to knock 20 C off of the RAM. Sometime, just the intake fan from an overhead radiator provides enough extra air to do the trick, but that are also simple fans for RAM from Corsair and others that do the job. They tend to be a bit clunky to mount, but usually it's not too difficult to get it done, especially if you have a water-cooled CPU.
It is generally not absolutely necessary, but you will get better life from the memory and possibly some additional speed if you are overclocking.