If I'm reading this correctly (that's not certain...), I can think of only one 'good' reason for having a separate, common, channel for Fx destined for the Master. During the mixing, it may end up desirable to have a channel not going thought these common Fx, or at least switch between in/out to compare. If sending to a common Fx, it would be possible to 'dose' the amount of Fx for each 'send', which would no longer be possible, individually, if the Fx is only on the Master. If this individual control is not required (overall compression, for example...), having this on the Master alone could make sense. There's nothing wrong with using both notions, though: route all channels through a common Fx channel, to 'dose' them if they require tweaking, and have global Fx on the Master as well, knowing that these will affect everything, together. I'd use both; for instance I have my drums in a group channel, and apply individual Fx to each drum element, but also a NY Drum Bus Glue compressor on the drum group channel. I have another NY Glue compressor on the Master, too. My answer, then, is... Both..!