That completely depends on what this 'opinion' is and how it's formulated. Unlike what some people are blabbering about here in this thread you can't just -, or + rep for that matter, people because you feel like it.
If someone says "Sakura is a worthless piece of crap" well frankly if that person gets - repped for that it's justified. It's an opinion, but it's practically character bashing. However if someone says "Sorry, but I just don't like Sakura" well - repped that person for that comment can be considered rep abuse as it's a personal opinion whether you like a character or not and you aren't harming anyone by it. - repping for that pretty comes down to you forcing other people to accept your subjective opinion about a character.
As said if you find a - rep unfair, report it. However in such a case it's also advised you aren't - repping people yourself frequently. I've seen many cases that someone reports a - rep, supposedly unfair, while they are giving out people - rep for the exact same things.
In the end I find - repping people either way childish, regardless of the situation.
