Different recipes will be better or worse with main hand versus off hand.
By way of example, if you try to make hempen thread with your main hand, I believe you only yield two per synth. If you use your offhand, you yield a dozen at the exact same materials and shards cost.
For other things, you can generally assume a true RP preference. By that I mean that you probably wouldn't use a spinning wheel to sew together an inner hempen dalmatica, you know? You're going to be using a sewing needle. By the same token, if you're making cotton yarn, you're going to do it on a spinning wheel.
Realistically you would probably use a loom to make hempen, cotton, or canvas cloths... but I guess they didn't want to put in tertiary tools? Better this way...