If you can afford to develop your business plan and prototype after hours and on the weekend--- while keeping your day job---do it.
If you have a little bit of startup capital, assume that you will never receive another dime.
Be realistic about what it is going to take to feed your founders. Can they live on the amount in-hand for one year? If not, don't quit the day job.
Our business was born of four founders, all of whom were fairly mature in their careers. It felt like we were making big sacrifices by paying ourselves approximately 60% of our former incomes. Still---the combined amount required was considerable. With what we raised we could have paid two of the founders full-time salaries for a year. But we all went in full-time, leaving us with only 6 months running room. It wasn't enough.
Six more months would have afforded us the opportunity to complete a pilot we began at a financial institution---a pilot with great promise.
So why did we decide the four founders should quit their day jobs up front? A number of reasons, including a strong belief that, to make the most of our combined energies, we must all be 100% committed to the effort. That creativity would best flow if we could all work together in the same room. There may have even been a "fun" factor. All wanted in on the excitement of rapidly building something from the ground up.
What is the take away? Don't let irrational exuberance into the equation. Plan for the worst. If you can't support your founders for a year with what you have in-pocket, think hard. Stick with the day job or find other creative ways to support yourself while you build the company.