Is Quicktime always pre-installed on the Macintosh OS? Does the Mac come with Quicktime pre-installed?
If so, what OS version (minimum) could be expected to have come with it pre-installed?
Every version of Mac OS X has QuickTime. QuickTime Pro is/was an optional paid extra, which included extra codecs and additional functionality.

The situation with "classic" Mac OS was rather more complicated, but I'm guessing you only care about OS X.