the best thing is to groom your horse daily, and check his body over for cuts,scrapes,fungus etc. grooming also helps distribute the oils to make him shine and prevent skin conditions.
However....i RARELY touch my horses tail as it causes the tail to get breakage.
The only time I touch his tail is if he is going to a show or clinic and I condition and spray detangle in it thoroughly and Finger detangle it.
Then after the show or clinic. I shampoo his tail to get rid of all the silicones I put in it (silicones just coat the hair and don't allow moisture in) and condition it.
And then leave it alone! (Dillon has a long thick tail and everyone asks about it ...they are shocked when I say LEAVE IT ALONE! LOL