Spacing is only for voltage and on inner layers is 0.25mm for up to 500V. It all depends on the design and complexity, and as Kabaleevisu has said more is better if the design allows. For a complex digital board with numerous voltage planes you have to use the minimum spacing to get the power distributed, on a Power board design the components are larger and generally spacing is less restrictive so a larger gap is used (as is also heavier base copper weight).
What sort of design is it?