If you speak at all I think you should say what you think/believe to be true. Otherwise of what use are you to yourself or others?

I agree, but I always try to do my research first unless I am asked for an opinion. Unfortunately, even when I have done my research there are many who would not believe me.

probably not much use

Not always so. There are times when bending the answer or outright lying is necessary.  Consider a job interview.  A common, if inane question is, "Why do you want to work here?"  The truthful answer is, "I need a job."  This is not appreciated by the interviewer.  Or when one's employer asks, "Do you enjoy working here?"  A wise employee will affirm so.  In truth the answer is most likely, "No! I hate it and I hate the customers and you are miserable jerk to work for."  Students in school face similar circumstances.  Or kids with angry parents.

The information that I give you will sometimes be balanced by my judgment of your right or need to know.  

Scholastic methodology involves a process of informational leading and knowledge exchange.

But I agree that saying what you think to be true is a good general principal.