Depending on why you're getting a DNA test for your horse, the results might not be totally accurate.
If you're doing it to verify your horse's parentage as a requirement to register it in a certain breed's studbook so you can register your horse, then yes, it will be worth it, assuming one or both parents have already been registered.
But if you're doing it because your grade horse's parentage is unknown and you want to find out what its dominant breed is, the test is less accurate than it is for dogs, for example.
Dog breeding has been a part of human life for several thousand more years than horses, and more accurate records have been kept longer, so a random dog's DNA will be more accurate today.
Horse breeding has been more accurate with records kept for the past several hundred years for some breeds, like for racing, but not so much before that, as well as some gaps during WWI and WWII.
Plus, the results of the tests are really based on what other people who tested their horses, attribute their parentage.
So, for example, you might do a DNA on a Standardbred, and it might say "50% Morgan, 20% Arab, 20% Percheron, 10% Andalusian". But some other person doing a test on that horse's full brother might turn out "40% Thoroughbred, 30% Standardbred, 20% Morgan, 10% Akhal Teke."
Just because Thoroughbreds came from Arabs and a few Akhal Tekes, during the past 300 years, and the Morgan genes came from both, as well as from Percherons and other drafts.
