Opps - I didn't read the question carefully. I agree that you should check out the wikipedia link
If you can find a copy at your local library, the situations for applying different matrices can be found in this book (which I think is a good introduction to molecular evolution):
http://www.amazon.com/Fundamentals-Molecular-Evolution-Dan-Graur/dp/0878932666
Does this help to choose a model: http://en.wikipedia.org/wiki/Models_of_DNA_evolution#Most_common_models_of_DNA_evolution ?