I could give a flippant answer by saying that all of them can be useful given the proper circumstances.
That said, on a 27-speed road setup, I would probably as a rule of thumb, avoid the last two cogs if not last three while in the big ring and the first two if not three while in the small ring so that's purposefully putting 6 combos in contention. However, if you really have to enter those combinations then do so but try not to stay in them any longer than you have to. Now different cogs and chainrings sizes will of course give you different combos and thus different usable gears as well as overlaps but just as an example, here's what the gear plot of my 27-speed MTB setup looks like. Note that the lighter the colour of the gearpoints, the more contentious they are.
