Using a "Find us on Yelp" link or decal is an unobtrusive way to encourage customers to leave feedback on Yelp. This message doesn't ask your customers to do anything, it merely reminds them that they can find more information about you on Yelp. If the happen to leave you a 5-star review in the process, so be it :)

Don't fear more bad reviews as a result of this link being available.

Let's be honest, if someone wasn't happy with their experience they either have already left a bad review on Yelp or they aren't likely to visit your Yelp page or restaurant again. In either case, having a link on your website or a decal by your entrance isn't likely to encourage someone who otherwise wouldn't write a review to go to Yelp and write a poor review.

So while you should never ask for Yelp reviews, you should be using signage on your website, facebook or twitter, or in your restaurant.