One thing nobody's mentioned is that it's possible to damage a triple lever shifting a double, if you manage to over-shift it, putting extreme tension on the cable while the derailleur's hard against its stop.
Not a very likely scenario, since you generally don't want a bigger ring than your big one... but it's a possibility. Or, you could set the shifter up so the cable goes slack on the full downshift. Not sure which way works better.