Ok, we all know that we need to shadow so that we can put it on our application and dental schools like it - but shadowing should be more than that. It should help you determine if dentistry, in fact, actually interests you. Shadowing early and often helps express sincere interest in the proffession. If you havn't had much shadowing experience yet I would suggest getting some as soon as possible to help you find out if you even want to be a dentist. What if you go in to shadow next summer and realize you would hate practicing dentistry? You would have wasted a year.
I imagine if I were on an adcoms commitee reviewing an applicant claiming to have been interested in dentistry since his freshman year in dentistry I would question his motivations. Are you interested because of what you've observed or because you've heard they make good money and don't work on fridays? Those are just my thoughts.