there's potential pros and cons to any method, so you really just have to try things and see what works for you.
1) some people add them after primary fermentation is complete, with the completely logical argument that fermentation produces CO2, which can have the effect of scrubbing out hop aroma as it bubbles out of the beer. Therefore, you want to add hops after the fermentation is done.
2) some people add them with a small amount of fermentation remaining (i.e. 80-90% of the way to final gravity), with the completely logical argument that adding dry hops will introduce some oxygen, so you want to add them at a time when the eyast are still active and therefore most likely to consume the oxygen and prevent oxidation of the finished product.
3) some people like whole hops, with the completely logical argument that they are less processed and have better aromatic properties.
4) some people like to use pellet hops, with the completely logical argument that whole hops absorb a lot of liquid that you can't recover.
I could go on, but hopefully its clear that there's a lot of very logical arguments for a lot of variations on dry hopping.
I will say I can't agree with OldWorld that dry hopping isn't worth it, but that's the beauty of homebrewing. I think its worth it, so I will do it where appropriate, which for me is quiet often. OldWorld thinks it isn't, and therefore won't.
