Jetrell Fo said:

So would it be safe to think that one wanting to do this using various sources (i.e. deleted scenes and such) should probably try use matching audio source quality to keep the output as consistent as possible?

Holy Cow! it is not rocket Science, deleted scenes are not going to be as good of quality as the movie source, but when the movie source is 5.1 and the deleted scene is 2.0 this guide will show you how to make that crappy 2.0 into an acceptable 5.1 to mix into your edit, that is ALL this guide does, no more, no less.

And it is NOT about polishing a turd as you put it, it is about keeping an edit predominantly true 5.1, which would probably be about 99% of the edit, aside from the upmixed deleted scenes, and OBVIOUSLY you want to use the best source you can, but when it comes to deleted scenes you are at the mercy of what is available to you, are you not?