Just got this board and am working on fitting everything into it.  It has a ATX_PWR_6P connection on the MB to provide additional power to the PCIE slots.  I will only have powered graphics cards (980ti, 1070). It uses one of the 6 VGA power connections on the EVGA 1000 PSU (which I prefer to use to go directly to three cards). Plus the cable is awkward in my case...
 
Do I need to connect this?  It says it's only highly recommended in the manual...

Nope.

Thanks for the quick response.

No problem.

bcavnaugh
With Two or More I would.

 
This

I've found that even with a single GPU along with other non-GPU PCIE cards that it is a good idea to plug in that supplemental power connection.
 
On my X79 Dark and a single GPU and a single discrete RAID card, the board would fail to full POST 80 to 90% of the time without that supplemental power plugged in. I've continued that practice with the X99 as well, and havent had that issue yet.

Taken directly from the x99 ftw k manual...
 

The 6pin PCI-E connector present on the motherboard provides additional power to the PCI-E slots, rather than pulling it all from the 24-pin main power. It is advised to plug in this connector when using SLI, especially with higher-end graphic cards. It is strongly recommended to plug in this connector for 3-way and 4-way SLI, regardless of GPU.

The OP isn't running SLI.