In using GTX 970's in SLI, Is it standard to put thermal paste on a GPU like the CPU or is it ok if I don't put any? Will there be a noticable difference and Can I use the same thermal paste that i've used on my CPU?
If you did not remove the cooler on the gpu, no you don't need to change the thermal paste.
If you did, then you might as well clean it and reapply the thermal paste.
If you did not remove the cooler on the gpu, no you don't need to change the thermal paste.
If you did, then you might as well clean it and reapply the thermal paste.
There's no "might as well" about it. ALWAYS clean and reapply thermal compound when removing a heatsink. Failure to do so will introduce air bubbles that will cause thermal problems.