I buy my paint on account and have 30 days, so it's really not my money yet. I usually don't ask for a down unless its a big job, most of mine are under 2 weeks. Might do 50 % half way .
Sometimes I have had to wait, but in ~30 years have only been burned a couple of times, and that was by contractors, not homeowners.
Everyone has a way that works for them. Personal presentation, relationship with client, etc all come into play. Do what works for you.