Be sure to check the rules of the show before you do anything else.
If you do decide to purchase them, make sure to get them in your horse's coat color. You'll want them as unnoticeable as possible.
I'm not sure that you said which discipline you show in... But, in regards to Hunters, the whole point is to exhibit a conformationally correct horse that is amicable and a pleasure to ride. Ear plugs allude to the fact that your horse is 'less than a pleasure' to ride, and that he is either easily distracted or spooky.
Some of the jumpers where I used to lesson would use ear plugs, particularly during the winter when snow would be falling from the roof of the indoor arena, and make loud scary noises. I don't think any of them used them at shows, though.