If it is a paid job in a dental office - then yes, accept it! From personal experience, I know that there is a lot to learn about practice management from a clerical perspective. You will gain a unique insight into dentistry. Furthermore, you will make some awesome connections with other dental professionals in your area. If you are wanting to get more clinical experience, you can probably volunteer on Friday's (assuming your office is M-Th). Best of luck!
Def do it. You will get a good letter of Rec from the Dentist. But make sure you ask him if you can also learn how to assist- even if it's little things. If the office does any of their own lab work, try to learn that too. Make sure you really take advantage of working there and learn everything you can. Just doing scheduling or filing isn't a big deal.