Experienced pros spend serious time defining default settings for a project. The right sized artboard , the right color model, the right units of measurement, are all essential to create projects in Illustrator.
For instance, for printed projects, youll likely want to define page size (and rulers) in inches, but stroke thickness (for image border, for example), in points. For web projects, youll want to define your page size in pixelsand use a size appropriate for the kinds of web graphics youll be designing.
That said, youll always end up adjusting your default settings as you work on a project. The nice thing about Illustrator is that you can change things like units of measurement or page size at any time, and your document is automatically associated with the new parameters you define.