I'm not really sure why you would buy a scope to do what is essential mixing/MASTERING.
-- Vow3ll
Well, you do see an awful lot of them in mastering transfer desks back home..."back home" being Nashville.
The main check for it is to see if there's anything that's so out of phase in the low end that the engineer needs to go back and perform some bass summing to correct that...before it either 1) wrecks the cutter head, or 2) cuts a lacquer that's unplayable. Now, for digital, it might seem that an o-scope isn't useful, but I also use mine when doing various stereo manipulation methods (like expanding the stereo field beyond the L-R between the monitors) and to check if there's something that's creating a spatial "lump" that needs correction so that the stereo field can feel properly centered. Mastering really is something of a "black art", though...the sessions where I did some of that while learning were a revelation in RElearning how mastering sessions work and what's essential for them, coz classroom teaching and/or textbooks will only get you so far.
But if I've got my ears, why do I need the scope? Well...your ears are only going to sort out the basic stuff if you're juggling 15-20 channels, so the scope provides a second visual check. If you see a problem, go back to that timestamp and fix it. Simple.