Personally, I'll trim the exterior fat from really fatty pieces when I make it. US meat must have less fat hanging on it.
The silverside cut (bottom round) has a large layer of fat on the exterior when I see South African channels making biltong.
I don't care for lots of fat on the biltong. The intermuscular fat is fine to me. My friend's father loved the really fatty pieces. The fat is actually better than I first guessed. I vacuum seal and freeze my biltong as soon as it's dry, so rancidity isn't a problem. Plus, it eats quick. I make two batches a month just for me.
I have made biltong from eye of round, bottom round roast, London Broil and chuck roast. My favorites being the eye of round or the bottom round. Bottom round roast is cheapest near me.
Apparently in country biltong is made from beef, exotic game, goat and fish.