By no means is it a must!!
I saw a rider at Paint World, that had a simple saddle, with no silver on it, and a really nice clean shirt, and matching chaps. No bling!! And to my knowledge, placed quite well.
If your good, and do the class calmly and neatly no amount of bling will win you the class.
I wear all the bling and a silver plated saddle, but only because I got lucky with a trainer who's my size, and gave me a bunch of her old show stuff from her 4-H years. Plus, I think it's fun.
At the show yesterday, I decided to not show in all the sparkly stuff for once, and wore a nice olive-ish color button down, black pants and chaps. I showed in my schooling bridle which isn't decked out in silver, and we still won our classes.
I had a long talk with the judge afterwards, about his opinions and how he judges. He said as long as you've got your patterns right, everything clean and neat, smooth, and correct, that's what will get you the better placings. Not the glitter. I just show locally at the moment, and honestly I wear the glittery stuff because I had access to it for a good price, and it's fun. But it's not required to do well, and if you get judged otherwise, that judge needs to rethink some things.
Edit: Sorry the pictures so huge.