I steam the bag which I reuse over and over. I'm not convinced that steaming is necessary for a new bag. Probably just gives you the warm fuzzies. If there's leftover organic deposit in a seam in an old bag, steaming -- unlike sanitizer -- should kill everything there.
Hmm, thinking about it now, I probably should start using a piece of cheesecloth to avoid the hard-to-clean bag seam.
I don't use the magnet trick, though. Dropping the bag quickly in towards the tail end of fermentation has worked fine for me. The comparison I have is hanging it from the keg lid, using fermentation to purge the keg, and then filling the keg with closed-loop transfer (more or less isometric to drop-by-magnet). Can't say I notice a difference between the results for the techniques, which leaves me to think that quickly opening the fermentor to dryhop is fine, with the only annoyance being that you don't have a long window for doing it.