I have Poser 4, 6, 7 on a portable drive so I can use them anywhere. I have Poser 9 installed on my laptop - as far as I know it has to be installed, unlike earlier versions - so I, er, can't use it everywhere. ;) 

If I wanted to, I could also install Posers 3, 5 and/or Debut. But I don't want to. :D

I generally go to Poser 7 first when I want to make a picture. Poser 9 has advantages that I'd like to use, not least rendering in LuxRender via Reality 3, but most of the time I find it too annoying for a variety of reasons that would probably go away if I persisted with it, and it gets left unopened.

Poser 6 has advantages in a couple of areas (cloth room behaviour and conforming to some older figures), so I keep it handy for those occasions.

I use Poser 4 only during content creation now. If a "lowest common denominator" prop or figure works there, I expect it will work anywhere (thumbnails and bump maps excepted, but I can take care of those easily enough).