Should I get wider tyres/tires for my tour if I'm not going to go off-road? I'm going to do a ten-day long tour down the west coast of New Zealand. I'll have some luggage in rear panniers, but no tent (staying with friends and in backpackers accommodation). There'll be a fair bit of uphill and downhill on some days. But I don't forsee that I'll hit gravel - I'll likely stay on sealed roads. Of course, it may rain (as if that isn't obvious).

My bike is a steel framed road bike, and I currently ride with 23mm tyres. Because it's an older bike I can fit wider tyres on, which some recommend for touring, but I wonder if it's worth it? In the context of my tour, if I went to 28mm tyres what would the advantages/disadvantages be?
I have seen fully-loaded tourists ride successfully with lightweight racing bikes (of years ago) on what were then called 7/8" tires (which is 22.2mm).  I've also seen them lay their bikes down in the parking lot at lunch and have the tires explode.  (Don't ask me what mechanism was involved, but it happened at least 3 times in a week among a group of maybe 20 on the lightweight bikes, among a larger group of about 200.)

As a commuter/tourist I favor wider tires (I currently ride 35s), for the slightly smoother ride, better handling on rough pavement, less frequent airing up, and better weight-carrying ability.  

But the main question in your case is whether the tires can simply carry the weight, at full pressure, without compressing too much.  Load up your bike (with dummy weights if necessary), sit on it (or have a friend of the same weight do so), and see how the tires look.