In the original Master Cleanse plan, laid out in the booklet by the creator of the cleanse, Stanely Burroughs, he says it's okay to occasionally incorporate peppermint tea into the fast. That's all that's been said on it, "officially" and there are various opinions from cleansers about what else can and can't or should and shouldn't be included. The laxative tea has at least 7 or so different tea varieties added into it, and he does just say "laxative tea" in his booklet anyway, so it seems to me that there is some lattitude on this issue.
Personally I don't see how occasionally adding in additional herbal teas would provide any great harm to the cleanse, but to be safe, I've really only used a "liver detox" tea once in a while in my past 14 days of cleansing, along with an occasional peppermint tea. It's your cleanse, so do what you will with it. That said, most who become committed "cleansers" seem to keep fairly close to the formula just to be safe.