I have never given a thought to fork tines up or down, but plenty to the left-hand/right-hand issue.

The American table manners I was taught as a child dictated that you hold fork in left and knife in right ~ just as they are placed in the table setting ~ to cut your food, and then to put down the knife and transfer the fork over to the right hand to pick up the pieces and shovel them into your cakehole. Oh, and you're only supposed to cut one piece (of meat, or pancake, or whatever) at a time, so you are supposed to be constantly passing your fork back and forth from one hand to another.

I was successfully conditioned to use the prescribed hands for cutting and eating, but was never cured of my propensity to cut off multiple little bites to minimize the hand-switching. I am sometimes even so defiantly ill-mannered as to slice up an entire stack of flapjacks at once!

It was many years before I learned that Europeans endorsed the much more sensible system of keeping the fork in the left hand full-time, both for cutting and for eating. Makes much better sense; I use this method sometimes when no one's looking, and during the two weeks out of my entire lifetime that I spent in England and Ireland, I forked with my left hand exclusively the entire time.

I think I keep tines-down, unconsciously, most or all of the time. That's how you hold the fork to pin down whatever morsel you're cutting with the knife, right? I don't believe I ever turn the fork over for any reason when I'm not switching hands.