I am not a believer that comp titles are necessary for a query letter. While they can be helpful for an agent to see where you see your book on shelves, putting in the genre is just as helpful. I’ve also found that comp titles can hurt you if you name a book or author the agent really didn’t like. It certainly makes for an easy rejection.
However, if you do plan on using comp titles, because you know the perfect ones, it isn’t enough just to toss them into the query. In fact, in my mind, adding comp titles can sometimes be more confusing than they are helpful. If you are using comp titles it’s essential that they accentuate the blurb. In other words, if I see a comp for Sleeping Beauty meets To Catch a Witch I need to be able to see in the blurb how those two stories come through, otherwise you’ve just confused me.