First, you need to comprehend the nature and size of the project you will be modelling. If the idea is mainly in the form, then you need to focus on the base material that will show best your form, regardless of how it shall look from the inside.
Some other projects, like houses, for example, may require an architecture model that shows the design of the facades and the inside layout. So, here you will be dealing with more details. If it is a model for an apartment, for example, then the inside is all that matters.
Urban design models are a completely different case. You will be modelling entire neighbourhoods so the level of details required for the architectural elements in the model is much less.