I have to say that I dont feel it obligatory either.  Sometimes I do, sometimes I don't.  There is no rhyme or reason to that.  I just finished a bryce tutorial dvd.  I did not create all of the models in the animations...most were purchased from here, there, or yon...  If someone asks me where they came from...I'd tell them.  I just dont feel that I should have to.

my .02

:)

BT