Well, more than 100 MILLION books have been published in modern history, so there's no way you can be fair and give them all a chance or "the benefit of doubt" without making quick judgements based on... their covers.
Hi Devita
1. To answer your question literally, I always judge a book by its cover. I only read novels and if the novel has not won any awards I won't buy it. Any book that has won an award e.g. the Booker prize, will advertise that fact on it's cover.
2. To answer your question metaphorically, I never judge a book by its cover. I always like to ask questions when presented with any superficial information, unless someone else has asked the questions for me (see 1).