It's simplest to let nVidia control things, but you CAN google individual games settings and see what others who for some reason enjoy testing every single setting on/off/etc, and see what is optimal, and then lock those settings in each individual game yourself. The overall improvement in performance/appearance has really shrunk in doing so in recent years IMO, but old habits die hard. It's still good to go through your settings carefully IMO, whenever you reinstall nVidia drivers it always flips SLI to OFF, plus if you have a Gsync monitor it's a good idea to manually check those settings as well and make sure they are where you want them.
SLI with AH, as Challenge said, no profile, and even creating one has never yielded large, if any, FPS improvements for me, with AA cranked or mid range, it's never seemed to matter much. 680 SLI, 970 SLI, and 980 SLI, I just don't bother with it now. Unless you play a lot of AAA big name titles that get the attention of nVidia and the developers regarding multi card/sli/xfire setups, it's better to put your $ into the single best card you can, so far as AH goes.
SLI does make quite a difference with an optimized profile, especially in 1440p and 4k at high detail levels in popular titles.