@CJ and cobhc:
I think the "reverb on the whole song to make it more coherent" idea comes from a pop and rock background where the music is more open, slower and has more "room". Also it is about using very short verbs that are not noticeable instead of the huge "Roman Cathedral of DOOM" setting.
Try this:
1) Take any of your songs, preferrably a slower one with some "space".
2) Set up 5 aux sends with different reverb plugins (it works best when you use different plugins and not just the same plugin with different settings).
3) Then choose presets of different sizes from very small to large on each reverb.
4) Solo some of your instruments (some drums, vocals, guitars ...) and add some of the reverb to them. Make sure you can hear just a bit of reverb when soloed, but not when you have full playback. It doesn't really matter what reverb program you use on what instrument for this little experiment although I'd put something with a little predelay on kick and snare if you still want the snappy attack to punch thru really hard.
5) Now link all the 5 reverb channels so you can switch them on and off with one mute button (in Cubase you have to link them for that, I don't know about other programs).
6) Listen to your playback with reverb - then mute all the aux channels and listen to the playback without the reverb. A/B back and forth and you'll usually notice that the mix gets a slight bit fatter and "rounder" when all the small reverbs are on.
I know that this is not the same as putting reverb on the master bus, but the idea behind it is the same: you want to add a space to the recording that is "feel-able" but not "hear-able". I prefer the method above to the "verb on th 2bus"-method, but both work. It all depends on the music material because sometimes even some reverb on bass and a lot of verb on kicks can sound killer (I am just thinking of the situation when you walk across a festival site and you hear the mainstage from 200-300 meter distance and there is just a drumbeat and a overdriven MM Stingray through Ampeg stack playing and it just sounds washed out but thundering and great, haha!).
I don't think reverb on the whole mix has any use in speed, thrash or blackmetal, though.