Yes, book ahead if you for three reasons. First, early June to the beginning of August is considered high season for Italy, and everyone goes, which means everyone needs a place to stay, too. To avoid traipsing around looking for a place to stay (and ending up with sub-par, expensive accommodations) look at websites like Hostel World to book in advance. Secondly, since Cinque is a rural destination, there are less accommodations than, say, Rome. If you book in advance you won’t risk being bed-less for a night! Thirdly, you say “we’re going” so I assume you and another person (or persons) are traveling together. If you want to stay together, definitely book in advance. Otherwise if you wait, chances are you’ll end up staying in different places. It’s easy enough to find a place to stay when you’re traveling solo but if you have more than two, I’d book in advance during high season.