The truth is...people will respect you more if you just be yourself and stand up for yourself. No one is going to get mad at you if you have an opinion different than theirs...unless that person is close minded.
I don't have a problem saying how I really feel about something. I do tend to stay away from politics and religion because it can be so polarizing...but even I break that rule sometimes.
Last year my boss made a decision that I didn't think was right or fair. I could have said, "Oh well, he's my boss, what can I do?" But instead, I wrote him an email, in which I disagreed with his decision and told him why. I could tell he respected me for it and he changed his mind and revised his policy. I also got a bit of a merit raise later on.
On a side note, I read an article that stated "people pleasers" on average don't make as much money as their more opinionated coworkers.
I should also point out that it can be hard to be around people who never voice their opinions because you never know what they are thinking. You never know if they really agree with you or if they are agreeing with you for the sake of harmony.
Speak up...it feels good! You deserve to have an opinion just as much as anyone else in the world.