I agree with what doc toothache said. He said it in a very straightforward way, but it is true. It's far more important to have the stats (GPA/DAT) than it is about how many hours you've been working at the dental office. High GPA/DAT will likely help you get into dental school, whereas being a dental assistant does not necessarily ensure that you will. However, the GPA/DAT mainly gets you the interview, but you also have to seal the deal with your personality, individuality, maturity, and personal experiences so working in a dental office may help with that aspect.
So I will say, if you have low stats, I would work on the academics side of things. If your stats are competitive already, working as a dental assistant definitely does help.
I work as a dental assistant now, and the knowledge will definitely help in dental school.
As for becoming a dental assistant, landing one may take some time if the dentists are 1) looking for experienced DAs 2) usually you have to be X-ray certified since that is one of the important responsibilities of a dental assistant 3) they want someone who is long-term
Good luck though! I do think working as a dental assistant is a great idea, but you have to really look at where your weakness is in your application and fix that. People with high DAT/GPA get into great schools without having ever to assist.