One of the many problems with using V-blocks is properly locating the center, especially after you turn the parts over, as they will shift, based on the actual dimensions of the parts, and the V-blocks themselves. To get any kind of accuracy, you pretty much have to center up to each individual part every time - a LOT of extra work. With collet chucks, or a fixture, your centers will remain in the same place the whole time.
Regards,
Ray L.