Yes, in the beginning of flowering the plants will grow very quickly. Then sometime near the end of the 6th week, growth slows and resin production becomes the plant's main priority.
This man has hit the nail on the head. It's depends entirely on your space or time restrictions as it will double to triple in size during flowering. The longer you leave it in veg, the bigger the plant will be, the more budding sites it will have and ergo, the more you will yield.
Most recommend at least 12" but as you said, it is a matter of personal preference. They can be put into flower at about 4 inches but the yield will be rather pathetic.