In my opinion, it's fair to assume that when someone makes a statement, they are implying it is their own opinion unless otherwise stated, because in my opinion most people have at least, in my opinion, a basic understanding of the notion of objectivity, and because in my opinion inserting the phrase "in my opinion" or a similar (in my opinion) phrase in every case in where in my opinion it is technically valid makes a sentence significantly less readable, which in my opinion is not worth the extra ambiguity that I believe the phrase in "in my opinion" is intended to solve.