I find that yeast are happiest fermenting in Celsius. Theyre not very smart, and conversions are cumbersome.
I find that my yeast are happiest fermenting in wort! That way they don't have to do any math. They just have to eat sugar and fart co2.
I use both US Customary (though this is the first I have heard of the term) and metric. Linear metric to meters is easy, I can't get my head around kilometers though. Milliliters and liters seem simple as well as grams. The rest though????