I use the big-big method for two reasons:

1) That's the way I was told to do it when I was first learning to size a chain.
2) I've seen the results of using a chain that's too short for the big-big combination on the road.

The first reason I listed is probably the actual reason I do it that way because I've never even tried other ways, but for those who don't know and are curious about the second reason, the result is that when you accidentally shift into the big-big combination the chain goes into the spokes of your rear wheel, likely cutting a few spokes and possibly breaking either your derailleur hanger or even the derailleur itself. I don't want to do that again so I use the method that I know will avoid it.