I was told in class by a Japanese teacher that なるほど can not be used when talking with people above, but in the same lesson we listened to a CD (training material) where a student was saying なるほど to his teacher.
Here at work I often hear my Japanese colleagues saying なるほど when talking with customers (consulting). Can I do the same?
EDIT: A Japanese friend just told me she would never write it to a customer, even by email, but it is perfectly acceptable over the phone as an 相槌 (sound given during a conversation to indicate comprehension).