I used to think that the inner margin should be smaller than the outer margin in two-sided paper layouts, as in classical book layouts.
This is an example of what I mean:
This example was taken from this document: http://www.ntg.nl/maps/30/13.pdf
However, people in the printing house where I am going to have my book printed recently told me that I am wrong. According to them, the inner margin should be equal to or greater than the outer one, mainly because there is a small part of the margin that gets partially hidden in the binding process.
I know that I should add a small offset to the inner edge of the layout. When I am asking about inner and outer margins, I mean without taking that offset into account.
So, my question is: What are some most common or accepted rules about inner-outer margins ratio, when talking about a classical book design?
When I say classical I don't mean medieval but just not too modern.