For me, it usually depends on the color and customer's goals. If I know that I am not going to see the car for a while, and the customer wants to pay for the added cost of me applying platinum(or another sealant) then that is what I do. If I know that they are only interested in the best possible looks and I will be detailing the car again soon, I usually go with something carnauba based. As far as when I evaluate it, usually as soon as I see it. If I talk to them on the phone, I just find out the color, condition, and what their goals are for the car. The prep is a totally different story, usually decided on a case by case basis depending upon what the customer wants to pay.