Polite forum protocol : If you disagree with a poster’s response a thumbs down would generally be considered bad form other than the friendly competition mentioned by Jonathon. Stating why you disagree with a poster, supported by verifiable data and facts and done in a polite, constructive fashion is a great way to maintain a happy forum, avoid hurt feelings and flame wars.
Everyone has opinions. Opining on a subject and leading in with " in my opinion", is a polite way to voice an opinion on subjects that may have no right or wrong answer.
All of the above is just " in my opinion"