For me, and strictly my opinion, it all depends on two things.
1) What type of knife is it going on.
2) Who is my target buyer
I've only been making knives for a few years and I've not posted pictures of my work because it wasn't where I wanted it to be. But that is changing, they are getting to where I want them to be. Anyway, I use brass on a couple of knives I make but they are targeted to people who don't want to drop serious money on a custom knife but want something more affordable. Carbon steel knives are ones that I produce that can and do get brass guards depending on what type of handle material I'm using. I make a small semi drop point that trappers love for doing fine detail work while prepping hides to be tanned and while skinning. It's been my experience that trappers want something that looks nice but preforms it's intended function flawlessly. They want more of a working knife. I also use brass on a couple of drop points I do which are intended for hunters. Again it's the same situation, they want a custom knife, something that looks nice but preforms. If I really break down my costs, brass is cheaper than stainless material so that's why I use it on carbon steel knives. They come in at a price point that people who have always wanted a custom knife but won't or can't afford to spend a couple hundred dollars are more comfortable with.
If I'm making a stainless knife, it gets stainless for a guard unless the customer want's otherwise. If I'm making a stainless knife then it's more likely that I'm trying to get someone who knows something about knives to buy it. That's why my "higher end" and I use that term loosely, knives get stainless guards.
And for handle material, I use wood most of the time. But, it's been stabilized unless it doesn't require it. At this stage, I can't afford to buy stag, ivory, horn and the other high end materials. I have the luxury of having a lumber yard that deals exclusively in high end domestic and exotic hardwoods for furniture makers and musical instrument makers where I can go and hand pic my material.
I hope that made some sort of sense.
John