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Yes, you should/can, especially for longtail, which may have almost no volume in AdPlanner. I worked for an e-commerce site that had a few people hitting for a certain longtail term, but those customers knew what they wanted, converted at a higher rate, and spent the big bucks. It's also worth noting that Google gets 3.5 billion searches per day (and trending up) and 16-20% of those searches are brand-new, never been searched for before.