I've seen a lot of products become immensely popular before they even attempt to implement a revenue model (just look at Youtube). Obviously, there needs to be a compromise. Taking money from the beginning is sure to stunt the product's user growth, but waiting too long leaves you with the loss of potential money. What is the best way to handle the situation?
Also, transitioning into a revenue model can be tricky for many people. Does it require sufficient preparation from the beginning in order to successfully make the transition while retaining most of the users?
The answers will vary based on which revenue models people are talking about (advertising, subscription, etc.), so make sure to specify.
If you're attempting a 'project' and not a 'startup', still think about what kind of revenue models would work for the project. If the plan isn't to make money (ie. make something cool), don't kill yourself thinking of how to make money from your idea.