Well I did not work as a CNA before nursing school, but I can tell you that you should absolutely do it if you can... it certainly would have made my life a lot easier in the beginning of nursing school. Coming into nursing without any 'real life' experiences with patients was quite an eye opener for me. Although I am very familiar with procedures and medical jargon, and very sociable and rarely uncomfortable communicating with people, I actually found myself feel slight uneasy the first few times I worked with patients as a 'healthcare provider'. I don't know why but suddenly I was not sure how to approach people or whether or what kind of conversation I could engage in with people in various situations.
Also, you would be amazed how long it would take me to do a simple bed to chair transfer on a patient hooked up to oxygen and IV pumps... i must have looked like an idiot! If for any reason it is good to re-affirm your desires to enter nursing and give you valuable experience to pick up on your medical surrounding and how to effectively communicate with patients.
HOWEVER, my only caution is that many people who worked as CNAs before entering nursing school felt that they knew more than they did, or felt that they were more seasoned and therefore did not take learning experiences as seriously and ultimately struggled by the end of first year. Being a CNA is a difficult but rewarding job, but does not provide a proper education about how to assess and treat patients.