I’ve been using Cubase 6 x64 (didn’t even install the 32-bit version) since its release and the main thing I think is essential is to delete (“trash”) the preferences folder on each upgrade. I think this forces a re-scan of all VSTs and perhaps with each new version, the handling, particularly of 32-bit VSTs in a 64-bit host, is improving – I don’t know, it just seems that way.
A recent update to HALion 4 has removed one source of crashes on 64-bit systems, however that might be a bit off-topic as this is about Cubase hanging (as opposed to crashing) on exit. I found two utilities which may offer some clues, BlueScreenView and WhoCrashed, and have enabled crash dumps, but of course, since then, my system has been rock solid … 
(or maybe: 
).
My specific use case is very simple though, I have about 5 third-party VSTis, all but one of which are 64-bit, I don’t work with video (much) or have an exotic setup. If I try out a VST, when I’m done with testing I remove it … I’ve seen people with 100’s of VSTs, most of which they tried once, but never use … and unless I really, really want or need a particular VST, if it doesn’t come in a 64-bit version, I move on.
I still have a feeling the problem has something to do with closing ASIO or MIDI devices, and this is why the symptoms are so varied, because everyone has a different audio/MIDI interface. It may be compounded by the fact that some VSTis have MIDI inputs and outputs, but that’s just my gut feeling.