I generally advise largest four. This means that out of a 21-speed bike, you might end up with 12 or 13 useful gears, based on excluding the six on the big/small chainrings, and other combinations that end up being the same or only marginally (a couple of percent) different.

In the long run, though, it is very easy to get absent-minded about gears on a tour. If your chain has been cut to a length that is based on wrapping around the big chainring, then the biggest cog, without running through the rear derailleur, THEN adding an entire link.

You then shouldn't have too major a problem with chain lockup. Rather, you will just wear the sides of the gear teeth, and wear the chain so it has more lateral flex than normal. Even then, that wear should be minimal because the noise usually will alert you to the crosschaining and you will make the required shift.