I'm not sure how useful that would be... since Invoices are listed on the customer's Invoices screen are only for that customer. Account Class and Region is a customer specific field, so the order would never actually change.
The way that we are set up, we have different billing "Regions" (for lack of a better term)... and we Generate, Print stuff and send the invoices, but we have different return envelopes for each "Region" of subs..... For instance... Region1 = Area1 - Dialup Region2 = Area2 - Dialup etc... Each Area has a different return envelope (don't ask me why, my board wants it that way ... Would be easier to have the invoices printed in a per Region area, to limit the amount of manual sorting that would need to be done....
You could sort the invoices by anything you want when you bring up the list of invoices to be generated (under the Scripts menu, you can find a Sort option).