It is acceptable to say that you will need a few days to do the job right, or that you'll knock off a few bucks in exchange for their patience. It is never okay to imply that they aren't as important as all your other customers. Unemployment is boring. Try to find something to love about every job or project you do. Otherwise, find a job you love.
Life is too short to be bored or boring.
If you've been doing something the same way for years, it's a good sign you're doing it the wrong way. Maybe it's time to find a new and better way to do it. Your customers aren't asking you to dye your hair purple and wear your kid's jeans. You should question why you're still doing things the same way. Let's play it safe.