The only 'theory' I can find is Steve Hed's, which is based on aerodynamic of wheels. See this article http://www.slowtwitch.com/Products/Things_that_Roll/Hed_Ardennes_Gets_Fatter_3559.html for example. While this sounds like it only make sense for road wheels, I can't find any theory against it on other type of wheels.
Looks like the best combination would be of the tyre to be as wide as the rim, so the tyre will 'look' straight to the air, e.g. 23 mm tyre on 23 mm rim. There are also tyres with 'lip' that should cover the gap between tyre and rim to make it even smoother. If you put 28 mm tyre on 23 mm rim, the tyre will create a bulge that will be visible to the air.
This also makes tyre selection rather limited because rim sizes don't vary much, e.g. 23 mm rims only available a few years ago. Currently, I'm still using 19 mm tyres on my 18 mm wide rims. I'd love to get 23 mm wheels so I can use 23 mm tyres first because it's better, second because it's hard to find 19 mm tyres.
On Sheldon's page he also mentioned that many cyclist exceeded the recommended widths on his page without any problem.
About the rim being disc brake rim, I don't think it makes any difference because even if the rim was shallower, the wheel diameter stays the same.