I'm glad you bought this up, as I've had exactly the same thoughts recently, particularly with regards to minotaurs and human warband members.
Minotaurs are meant to be 25mm bases according to Warhammer Fantasy Battles, but in Warhammer Armies they are clearly on 40mm bases in the army shots. Also, have you ever tried fitting one actually on a 25mm base and ranking them up? Mine certainly need their personal space and will end up on 40mm bases.
With regards humans, they are meant to be on 20mm, but you may wish to put them on 25mm bases instead to be able to use them as Thugs in standard Chaos armies at a later date. As weismonsters mentioned, the interpretation of the basing rules seems to be at the painter's discretion.
For instance, I've put my Dwarf hero on a 25mm base in order to accommodate his freshly mutated wings.