It ain’t what you say, it’s the way that you say it!
If the examiner asks your opinion about something in part 3 of the speaking test, then it’s perfectly acceptable to say what you really think about the subject even if the examiner does not agree with what you’re saying. The examiner is only interested in the grammar, vocabulary and linking words you use to get your point across.
When giving an opinion, you will need to explain your position and expand your answer by giving reasons for your opinion. This will show the examiner a greater range of structures and vocabulary then just giving a short answer.
Look at this example from the Speaking Test about climate change:
Examiner: What do you think about global warming – do you think it's caused only by human activity?
Candidate: Well, in my opinion I don’t agree that global warming is entirely caused by humans because I think that it’s more of a natural thing. If we look at history there have always been changes in temperature so this is nothing new. I agree that we humans are speeding up this process, but I don’t think that is the only factor here.
(This view is a controversial one but the candidate has justified their opinion. The examiner will only assess the language produced to express and justify the opinion, but they may use follow up questions to probe the candidate more deeply on this).