Because I am on several other medications which are prescriptions, I would discuss most herbal ones before taking them.
I was recently taken off a few including calcium as they have discovered they are not good in all cases, and can actually do harm, so in future I think I would discuss them all first before taking them.
I discuss taking ANYthing with my doctor as so many seemingly innocuous things can interact with my necessary medication. It is not worth becoming ev en sicker just by not taking the time to ask questions.
I don't necessarily discuss them before taking them - I'm not on any other medication or treatments which they may interfere with. But any time I go to my GP I make sure I let them know anything that I am taking such as supplements or herbal preparations. I also discuss these with my dietician when I see her.