With my business background, I naturally start my creativity process on a concept with the business model in mind. I understand that it may (and probably will) evolve, however, it is important to brainstorm. At what point in your product life cycle do you consider your business model options?
If you care about actually making money from the very beginning (as opposed to just being bought -- which isn't likely to happen -- or just getting a tonne of users and worrying about it later), then consider your business model at the very beginning. Changes or problems with either your product or business model can significantly affect the other, so if you can focus on both from the very beginning, I can't think of many reasons why you wouldn't.
This book <a href="http://www.businessmodelgeneration.com/" rel="nofollow">http://www.businessmodelgeneration.com/</a> is an invaluable resource for entrepreneurs. I am constantly working on my business model canvas because it provides great insight and a clear overview of your start up.