quattro drive questions

[PJ <pajono@compsol.net>]

OK,
  Awhile back (2 weeks ago), I told my story about my recently purchased
86 4k csq. When I had it up on blocks and in gear, the front wheels
would spin, but the rear wheels didn't. I was concerned: this is
supposed to be perm 4wd, right? With the center or rear diff locked,
everything was fine, all four spun.

  I got both responses to my concern. Some told me my car was having
trouble, others said it was normal. Well, last weekend I bought a '87 4k
csq ($600 US 140k miles, beautiful, messed up rear quater), drove it
home (40 miles) fine, no noises. Put it on blocks, same thing! The rears
wouldn't spin unless the center or rear diff was locked! Now either I
had two quattros with the same problem, or this is normal.

Can someone set me straight? Why are they behaving this way? 
And while you're at it, can someone give me examples of when you would
lock the rear instead of the center? I'm a newbie to 4wd, let alone
qwattro drive.
Thanks a bunch!
Paul
=)