The adcoms aren't going to look up the number of hours someone has shadowed... they've got thousands of applications to go through. Most schools don't care, while some schools require shadowing, they really just need a letter from a dentist that you've shadowed... won't matter whether you have 40 or 4000 hours, documented or undocumented.....
Obviously don't discount shadowing entire, it does come into play IF you bring it up and develop it as a major "push" or point in your overall application. Like if you make it into a major selling point while in the process of whoring yourself to the adcoms, then it will matter. Otherwise it really is a secondary issue, if one at that even. My experiences from my 2 application cycles were that my lack of shadowing (0 hours shadowed) didn't matter one bit, it wasn't even an issue because not at a single interview was I asked about it (4 total). It did come into play with UNLV, I didnt' know they required a dentist-shadowing letter, so my application was never completed, so obviously find out what the school's individual requirements are.... but most schools do not care. Don't spend too much time getting hung up on the shadowing stuff... it's secondary.