So my gtx 970 has 2x 6 pin power connectors and my power supply cables give 6+2 pin connectors. I should be able to just snip off the extra dangling 2 pins, right?
Sure you can, but why would you want to? If you're concerned with looks, buy a 6 pin custom extension. Like others have said, you might need those 2 extra pins one day.
If you really want to, just make sure you cut them cleanly and try not to leave bare wire exposed where it might short something.
Seriously - don't be snipping wires with your power supply. You're going to have a bad time. If it really puts a black fly in your chardonnay then just wrap it down with some fancy electrical tape.
At the basic level the 'hurt' is the lost functionality for a trivial thing like "it looks tacky". The more theory 'hurt' being having two clipped wires floating around computer case. Again - if I always caution when mucking around with things like power supplies (ground wires or not). In the end it's your rig so go nuts if you feel the urge. I would like to see your before/after pictures to see how much "tacky" was resolved.
If I'm not mistaken, they gave a few sets of the exact same pci-e cable. I would clip the excess 2 pin adapter as close to the base of the rest as possible. I can put liquid electrical tape on the exposed part if need be.