Having served my mission in Japan I know that barley tea (mugicha in Japanese) is okay to drink. It was served at church socials and was a staple drink with the missionaries. It is simply roasted barley and nothing else. The taste is strange to a new-comer but is very satisfying once you are use to it.
Also, remember that in the wow it says. . .
10 And again, verily I say unto you, all wholesome herbs God hath ordained for the constitution, nature, and use of man. . . D&C 89:10
If you cook you herbs (make a tea-like drink out of it) then how does that make the herb bad? I think most herbal teas are nothing but harmless, if not wholesome herbs ordained for the use of man.
Clif