Typically, I polish after I clay. I use Megs QD as a lubricant, and am not quite sure if the QD would have any impact on the polishing process. Since I can wash the car in about 15 minutes, I figure an ounce of prevention is worth a pound of cure.
Since I'm not a professional, and I only clay my car about 3-4 times a year, it's not that big of deal for me.