Ambient Occlusion is often different across the surface of the model, so it usually needs unique UV space for every surface. However some parts of a model could have the same occlusion, wheels on a car for example, so you could overlap those UV parts to save UV space. Normal maps typically have a lot more reuse…
http://wiki.polycount.com/wiki/Topology#Principles_of_Topology Theory and examples for in-game models, vs. other kinds of models. http://wiki.polycount.com/wiki/Texture_Baking#Baking_Workflow Workflow for in-game models. Explains lots of the issues you'll run into for your model.