Ohh it's not the plaiting that's the issue
It's whether I have to plait him up or not lol.
Cos I get so annoyed with plaiting him up, It's so much effort cos I don't thin out his mane and I'm trying to get it to grow out a bit.... =P
As far as I understand there are no EFA rules regarding this in showjumping but out of respect to the people running the show if there is a public audience present like at an ag show then you are supposed to be well presented and braiding would be a nice idea. If it is just a local club show you might be going a little over the top but nothing wrong with over dressing :)
Like PP said, out of respect to the people watching, the judge and the show volunteers/managers, it is a good idea to braid. You look professional and it shows that you took the time to care for the way you and your horse turn out.