Just Thursday I emailed ARHA and asked a question on rules. My email was returned on Friday with a definitive answer. I suggest you email and ask the governing organization.
Bob and I just started working with cows. We rent a herd of 10 and a cow man from 6-8 on Thursdays. The cow man openly says he does not show ranch boxing (what I want to do) but does show in penning and sorting (different group, different rules). He urged me to ride Bob in a snaffle stating, “He’ll follow his nose better.” I said at 10, Bob was past the “snaffle age.” The cow man said, “everyone shows in a snaffle.” —ok, news to me, but I didn’t pay to argue with him, just work with his cows and learn about cow work.
The next day I emailed ARHA whose rules govern the Ranch Horse Association of Michigan shows where I show. I asked my snaffle v curb question. Finite, and without equivocation the response was: Bob, at ten years old, must show in a shanked bit and be ridden one-handed.
My trainer’s boss (the head trainer) is also a Ranch Horse Judge (among many, many other judging certificates or cards). When the question came up some time ago while I was riding Bob there, according to him, fast spins and long slides are not desirable in Ranch Horse. He didn’t say anything stronger, like forbidden or illegal, he said, [fast spins and sliding stops] . . .“Won’t get you more points.” He said a correct turn and a solid stop are what he looks for.
Again, I’d email the ARHA . . .their club; their rules. And me, being me, I’d forward their answer to the person who told you he’d mark a sliding stop higher.
I don’t show Ranch Horse Reining, so maybe it is desirable there.