The tail shows a lot on a horse, in dressage, a rhythm and swining will show in the tail - tenseness, tail wringing, tightly clamping it all show resistance and stiffness.
So, to emphasize the good or bad, it isnt braided, granted it's a trend - old old competitors used to braid in some areas.
The mane braids can be varied, and by examining the build of your horse you can emphasize the neck or detract from it with braids.
So, practice braiding at home a lot, it's fun, makes for a good feeling of accomplishment, and videoing your rides when braided can help you see what looks good or bad and you can change it before a competition to give yourself and your horse the better look.
(bouncy trotting horses with lots of "lift" do better with tight braids that don't leave much to 'bounce' and thickness or thinness of neck, upper muscle development or poor neck shape can be shown better if braiding complements instead of detracting or emphasizing faults.