Another option, though far less common, is dry hopping simultaneously with your yeast pitch. The argument against this is that volatiles will be blown off through the process of releasing CO2, and this is true to a point.
I had to experiment with this process when working at a pub that used spundigs for carbonation. Since the fermenters had to be sealed with the pressure relief set at a certain point to allow gas beyond that needed for carbonation to escape during fermentation, there was no convenient method of adding the hops post-fermentation. The only reasonable time to put them in the fermenter was along with yeast pitch.
After fooling around with it some, trying different amounts of dry hop, etc., I came to the conclusion that an additional 10-15% of the dry hop bill was required to achieve a similar effect in the finished product.
The upside is less effort and fewer times opening/closing your fermenter, every time you do so it is another potential contamination source.
The downside is that it takes more hops which means more $ and more junk in the fermenter.