Given that you have helped me on numerous occasions, you are well aware that I am still very new to recording, so my answer comes with this major qualification, purely from a listener's perspective, and just as an excercise in discussion. Having said that, I think timing between songs is very important, perhaps more so than one might initially assume. There must be a definite sweet interval, if you will, that is best suited to coupling any two songs. I recently read something in The Mastering Engineer's Handbook by Bobby Owsinski to the effect that the interval should be some commensurate measure of beats of the preceding song, if it were in essence to continue on instead of terminating.
I think I would be inclined to throw the last and first several measures of any two songs on a loop and listen to them continuously, while nudging the latter until it sounded as natural and effortless as possible. I also think I would fuss over this quite a bit. I mean I recently trimmed my contest entry's fade out by some four seconds after realizing that it was far too long and indulgent, which also kept the listener from getting back to the beginning while interest might still be reasonably strong. It could probably still be shorter.
Small details may not mean much of themselves, but they sum to something greater, regardless of the craft or discipline.
It might make for an interesting experiment to see if the result of a purely intuitive nudging process would correlate strongly with a more mathematical approach.
In any event, let the mastering masters now set the record straight. :smile: