I’ve worked at the back gate at a number of shows, from WCHR to local shows that state they follow USEF/USHJA rules, and occasionally get calls from the judge (via the announcer) to let a competitor know their tack/attire/something is the reason they’re eliminated or out of the ribbons. Those reasons are almost always:
-Forgot to pull polos/boots before entering the ring
-Sun visor still on helmet
-Visibly wearing radio/cell phone on their belt
-Black stirrups in the equitation
-Not following formal attire guidelines for night classes as is laid out in the show’s prize list
-OCCASIONALLY rider wearing boots/half chaps at a rated show**
-Illegal bit (gag, bit guards, etc) or noseband (flash left on from schooling)
Never has a judge commented to me about how a rider wears their hair, the color of their breeches (as long as it’s allowed in the rules), the shape of their bit (eggbutt vs D-ring vs full cheek vs loose ring) or the brand of their saddle.
That being said, it’s easy to watch riders walk up to the schooling ring and see who knows the unwritten rules and who doesn’t, which often (but not always) correlates with who is better prepared. A good judge (particularly at the local level or in a lower level class at a rated show) can look past less conventional turnout in the case of a good trip, and top-notch turnout will never mask a crappy round. My personal opinion is if i’m going to spend the money to show, I’d rather not risk giving a poor first impression, particularly if it’s a cheap fix like a $20 fleece shaped pad or a $2 hairnet.
**Edit: it looks like this is no longer a rule, so nevermind!