In order to keep a clean flow of your geometry, if everything starts together, you'd have to work quite a bit to make sure everything was clean where the major pieces meet. You're going to have to do that, anyway, but I find it best to not have to worry about it all throughout the modeling process.

In any case, ifyou use a cube as your starting point, it's best to round it out some, unless of course, you're making a Lego man.