"Recent news reports question whether existing scientific research support oral health benefits associated with flossing. The bottom line for dentists and patients is that a lack of strong evidence doesn’t equate to a lack of effectiveness." ~American Dental Association
So...everyone should floss their teeth daily but when is the right time? Before or after brushing? Flossing before brushing lets the brush remove any debris left behind. Also the fluoride in your toothpaste gets to areas plaque covers. But on the other hand flossing after brushing is thought to better help in fighting gum disease.
In truth—whatever works best for you—there is no rule! The important part in all of this is to floss at least once daily and brush twice.