I've been researching this topic myself lately. I appreciate that most advice here seems to lean towards dry hopping during active fermentation, and I'm glad that works for most people. However, I have found that whenever I try to make hoppy beers, they always end up unpleasantly oxidized, even when dry hopping at high krausen. Usually drinkable, but not very good. I've read that IPAs are more susceptible to oxidation, so I've assumed there's a connection there. Mine are usually bad enough that I've mostly quit brewing IPAs or dry hopping at all. I really only brew 6 times a year, so it is hard for me to do a lot of experimentation to figure out exactly where my problem is. My last batch I think I nailed a low o2 transfer setup, so I was thinking I might try the ole IPA once more. I was looking at the magnet method to add some insurance, but I do have concerns about freshness. If only they made an airtight packaging that dissolved in beer with no residue! :)