I have no idea what yall are talking about besides NES carts and while I wouldn't spit in the slot, saliva is acidic and helps clean the contacts. I wouldn't recommend it to properly clean them (as noted above, tarnish will happen eventually) but for a quick test on a 'new' cart before proper cleaning. I just use a q-tip, shove it in my mouth to pick up saliva, wipe off the contacts (qtip usually turns black or green, if it comes out looking just a little black/silver, it's already good to go as far as saliva is concerned), blow on it, and turn the shit on and play. My old carts from back in the 90s still work too.
NES was almost my complete childhood, and I still use this same practice on the carts. Never fails me. Also, try pulling your carts to the right after pushing them down, helps some sometimes if you haven't replaced your slot or do not have a top loader.
As for the SD Slot, what I did was took a dead SD card and made a pin/contact sander/cleaner with it. Just need to mount up some nice fine sand paper in the pin slots of the dead card.