I can't imagine NOT discussing my opinions with my children. It's called conversation.

No, not as long as you are willing to listen to their opinions and reasons why as well.

"Wrong" to discuss your opinions? Why do you doubt what you've chosen not to believe?

Who would it be? As long as you use age appropriate language and don't start berating them for having a different point of view or asking questions I see no issues.

I always tried to be truthful with my daughter as my parents were honest with me about many things. There were a few such as money that I understood to be the way they were themselves raised but other than that, they were honest and expected me to be as well. The fact that they would admit when they were wrong and apologize to their children had a profound effect on me. Another huge part that I did not understand until I left home was the emphasis they placed on reading. Sometimes as a child, I would ask a question, and they would tell me to go and investigate and report back to them on what I found.
So, the fact that they emphasized reading, admitting to and apologizing when wrong and to look something up, reason it out and decide for me what was or wasn't meaningful had a considerable impact on me as a child and now as an adult.

I personally feel this comes under education. I feel it is important to tell my children why I take a particular stand on various things. When I told them no about something they wanted I always told them why. They may not have agreed with me but they knew why I took that position. I also feel the same way when speaking to other people. To take a stand on something and not to state why I feel is presenting only half a statement.