It really does depend on a multitude of factors.
First of all your own tolerance of weight or bulk, your ability to actually carry out the repair and your own motivations.
Tour length. For a short tour a good review and service should be sufficient, unless something drastic happens.
You also need to think of the likely availability of your rear mech (or anything else) where you will be touring.
There's the need to consider your bodging skills to get you to a replacement.
There's group size. A large group could conceivably split a large amount of spares between them without too much hassle.
There's route and terrain and weather conditions. I've read many accounts of rear derailleurs getting banjaxed in heavy, heavy mud.
However, unless in out of the way places I'd be unlikely to think it necessary. People are generally friendly to people on bikes and lifts and assistance are usually readily available.
I'd be more likely to think that flexibility in plans is probably more important than carrying spares - allowing a bit of extra time to deal with the unexpected. Truthfully, dealing with hiccups on the road is often very satisfying and lives long in the memory.
There's Murphy's Law too - the very thing that breaks will be the one you don't have a spare for!
Finally, there's a danger that thinking I have to have a spare X can easily lead to thinking "I need Y & Z too". Often these thoughts can lead to folks never heading away at all!
There's a "disasters" category on CrazyGuyOnABike that makes for interesting reading - both from the point of view of what goes wrong but also how people deal with it.
As with all things touring, I think that what's between your ears and beating in your chest is far more important than what's in your pannier.
Good luck!