That is a common debate and my answer is it depends on the strain and type. Sativa plants tend to take longer to flower and grow slightly during flowering, while Indica plants tend to grow denser buds that stay shorter. I prefer to let plants flower until they are just about dying... they look like trees in fall with larger fan leaves yellowing and falling. It's my opinion and experience that you get the majority of the heft and flavor dring the final days of the plant... usually a week or two after the "optimum Trichome" method for determining "doneness". Indica plants mature faster and don't flower as long, so I don't usually let them grow as long as the Sativas as they will start to grow funny and start foxtailing, which is an abnormal type of flowering.
Anyhow, that's my opinion!