Naturally, every author wants good reviews. And although it's perfectly ethical to seek reviews, don't do anything to suggest you're expecting favorable treatment. If you succeed in getting lots of reviews, you can expect some negative ones.
"I see a fair number of books that I don't like, and I say so--including those sent to me as review copies," says Jane Corn, one of Amazon's Top 150 reviewers. "Anything else seems unethical to me."
You can safeguard yourself a bit by requesting that Top Reviewers not post a review if they simply hate the book. But it's the reviewer's call. Sometimes reviewers are willing to give prepublication feedback, providing valuable advice on fixing a book's weaknesses. Don't expect that, though, and don't ask for it.
You might want to avoid sending your book to reviewers who usually post harshly negative reviews, but don't shy away from those who offer frank criticism. These voices lend credibility to your book, Corn says:
Readers are smart. They can figure out who to trust, and those are the reviewers you want to reach. Always be clear about your willingness to have a fair, honest review. Anything else is self-defeating.