I usually wait until primary fermentation is done before dry-hopping. The reasoning is that the activity in the fermenter can "scrub" the hop aromas away partially defeating the purpose.
In a recent issue of New Brewer, they had an article about getting the most out of your hops. One tip was that some breweries will add their dry-hops when there is still some active fermentation happening. The idea there is that fermentation has slowed so your not scrubbing much and the convective activity keeps the hops in suspension rather than just falling to the bottom and sitting idle.
So I tried it on my last brew that was a single malt IPA with the hops an even 50/50 split amarillo/simco. I added hops on day 3 of fermentation when there was still some churning going on, but it no longer required a blow-off. I didn't rack it though.
I kegged at 3 weeks, force carbed over a couple days, and the hop aroma is amazing. I want to investigate this more and plan on doing a 10 gallon batch to do a side-by-side comparison, but I was impressed with the results.
I'm sure someone else will chime in, but I don't think the article was a misprint.
Primary: Spurhund Zunge (Berliner Weisse), C&C for C's (gluten free Pale Ale)
Conditioning: Pomapfelwein, Oaked Imperial Stout
Drinking: Juniper Pale Ale, Baltic Black Porter, Single Malt IPA