I turn away any job when the client refuses to pay anything up front. It sends a red flag. I also charge a scheduling fee which is non-refundable. I get 33 percent when I show up and begin work. Another percentage halfway through, and the balance upon completion after client is satisfied. There needs to be skin in the game for both parties as a measure of good faith. If you are dealing with a reputable company (did your due diligence, right?) why wouldn't you want to pay something as work progresses? We do this not only because we love to paint but we require cash flow to stay in business. There is not always 'money in the bank' as you suggest. It's tough these days. The suggestion buy 'Kim' 'Never pay a contractor a deposit' is nonsensical.
If the point of hiring a well established, experienced, reputable painting contractor is to secure the professionalism and trust suggested to be inherent with that choice, then I would EXPECT that professionalism and experience to include the ability to make the proper and correct calculations for labor and materials for a fixed price quote, and there should be NO reason for the contractor to put the cost burden of their miscalculation on the consumer.