Data Modeling?

Is it a 'norm' that DBA's use a data model (both logical and physical) to design a database? or Are data models mostly used as a 'post-design' afterthought, to document what is physically already implemented? In most shops that I have worked, we never used a model. And if they did have a data model, it was never up-to-date. The model was an after-thought for documentation purposes... It was done after the table was implemented in Production. Is there a good modeling tool for Informix 10? Is there any one 'data modeling' design class that is better then others?