For me I like to hold the open side to the right of my mouth, I feel like my right hand has more confidence in holding it closed and together
Only time I do a left open side sandwich eating is if I have to eat it in reverse after determining this will provide the most meaty finale. Nothing worse than that list bite just being bread - I always try to divvy it out to achieve the best happy ending
Here's some diagrams
This guy is performing the classic left side open to mouth
This next one is preparing to do right side of open mouth, you'll notice how much more happy and optimistic he is