If the photographer asks you to be a subject and you say yes, then you are indeed doing him or her a favor. But, if you cancel at the last minute, you're doing the opposite. We're used to people saying no to photo shoots. It's not going to hurt our feelings. In fact, it hurts much less to hear, "Sorry, I hate having my photo taken," or "I'd love to, but the next few weeks are really busy for me," than it does to show up at a venue with a bag full of stuff only to have it be a waste.