Yes, that's what I meant. Review anonymously, if it's allowed. If it isn't allowed, create a second account that has nothing to do with your name/nickname or anything that could possibly identify you (create an alternate email account under another name, if you can't hide your email in whatever service the fic is being hosted/reviewed at) and only use that to leave "unpopular" opinions. That way, your reputation isn't tarnished, but you can still speak up your mind.

Just be polite, don't flame, don't be condescending, and if the author still bashes you anyway, at least that way you'll know what that person is really like.