Whatever you do is your choice. I used to add some tea tree oil to peppermint flavored Dr Bronners. And when I could not afford that I used Dr. Woods Peppermint castille shampoo with the same ingredients(as Dr. Bronners peppermint soap) for like
8 dollars for a 32 ounce bottle.
I got it that cheap at a wholesale store in Queens, NY. You can find it for a few dollars more online:
updated by @andre: 07/11/15 11:39:18AM