I've never heard of this for MR or any other program. In fact, in my experience, one downside to booking Priceline is that you lose the ability to participate in rewards programs.

Last summer I booked a "3.5 star" property in Charleston, SC via Priceline. It turned out to be an old Holiday Inn in so-so condition at best. Terrible deal. I caught that Priceline actually booked me for $10/nite higher than the published rate. After 2 emails they did reduce my rate to the published rate, but I incurred their fee and was not able to get the points. I stopped messing w/Priceline at that time. Now, I have enough points to get rewards rooms, so it's moot for the time being.