For Cowboy action, I shoot the 357 brass, just because I want to. Many others shoot the 38 special. I worked out a deal with a local gun dealer here, but still paid extra for that 10th round. I dunno about the 9 magazine (.357) holding 10 38 specials. I suppose it is possible, but going through my loading manual, I would think the round would be out of spec.
I know my marlin is pretty picky about the round nose stuff and it has to be the right length for it to cycle properly. I use the flat nosed leads and have not had a single problem with mine. I looked at it this way, that 10th round gave me more barrel length. For the Cowboy silhouette shooting, the extra length gives me that little bit of flatter trajectory. That's my story, and I'm sticking to it.