Almost forgot to answer that question. I voted "It varies, but most often start-to-finish" and although that is normally the case there are many ways I can listen to C&Cs, depending on the nature of the score:
A) Some scores are varied, constantly interesting and at the same time short enough to just listen to every cue in one sitting - e.g. The Wind and the Lion, Empire of the Sun or Poltergeist - these examples all have the lenght of a generous OST, they have really no noticable portions of dull underscore and the pacing and musical development is naturally given.
B) Other kinds of scores do not lack the measure of variety and constant musical merit, but they might be just too long to listen to them from start to finish - e.g. The Lost World or The Empire Strikes Back - it isn't exactly a great act to find a narratively reasonable point to cut the C&C program in half, so I often do that with these kinds of scores. In some cases, the creators of the C&C releases already split the program, so that it makes sense.
C) On the other hand, there are many scores that partially consist of hot air and just need a few cues of tracks thrown over board - e.g. Masada, Deep Rising, 1941 or Powder - that's either because the C&C program has too many super short, irrelevant cues (the first two examples) or because it gets very redundant after a while (the last two examples). So I normally have a Windows Media playlist that excludes a number of short or uninteresting tracks or I even create an alternate OST program editorially, combining multiple cues to get a good pacing that the C&C program couldn't deliever.
There's a special case in which, when I find that the OST of a score doesn't represent the tone of the score properly, I created an alternate OST program for, such as The Ghost and the Darkness and The Prisoner of Azkaban. I mostly listen top the C&C program of these two, but I gave the alternate OST to some friends, because I didn't want them to get to know the score in the form of the lighthearted commercial representation.
In the end, at least for me, it depends from case to case in which form I use to listen to a certain score. I'm not always for the C&C and I'm not always for the OST, sometimes I think that neither of these two options can live up to the true potential of a score, so I try to create a better program. However, the problem of this kind of music is that there is no definitive way of listening to it outside the movie.