No it's not.  Unless you plan on doing any extreme overclocking like when LN2 is involved.

Sorry to resurrect an old thread but I am in the midst of troubleshooting random games crashing to desktop @ 4k and wanted to know if the supplemental PCI-E 6 pin connection is required for SLI. Manual states it is "advised" for SLI and "recommended" for tri or quad SLI/Xfire. I have a Seasonic SSR-1000PD PSU and it only has 6 outputs for 6/8-pin power, 4 of which I use for my SLI'd FTW3's and 2 are used for the 8-pin EPS/CPU connections. Kind of weird to not have enough connections on a $200 PSU but it also seems like maybe, just as the second EPS 12v connection, the supplemental pci-e 6 pin power isnt a requirement. Thoughts?

I would strongly recommend using the supplemental power if using 2 GPUs, and advise anyone using 3 or more to use it. The 6pin connector prevents the PCIE lane from over-drawing from the CPU or 24 pin connector, and can prevent damage to your motherboard. As bdary mentioned, this first became apparent in the X58 series, and we've included the supplemental power ever since.