Not sure about the latest cards, but SBA has not been officially supported by nVidia for a while now (starting with the first GeForce 256). Some card manufacturers may leave it enabled in the video card's BIOS, but most I've seen don't.
For SBA to work, you need to have enabled in the mobo BIOS, enabled in the video card's BIOS, and supported in the video card's drivers. Any break in the chain would cause it to not work.
With that said, I'd just leave it off. You're not missing anything.