To echo the other comments, the only time I've seen it make a difference was on international flights, and particularly on other *A carriers.

The most amusing for me was ORD-LAX-CHC, with the LAX-CHC segment on Air New Zealand. I had checked a backpack in the business class line at O'Hare, where it didn't receive a priority tag and was just thrown onto the conveyor, but ~24 hours later at baggage claim in Christchurch, it was one of the first items on the belt -- but now enclosed in a clear plastic bag with several priority stickers attached.

On the way back home (SYD-SFO-ORD) with the same checked backpack, it got a priority sticker in SYD and was among the first off the belt in SFO, but one of the last at ORD. And no clear plastic bag treatment either (the straps were well secured both ways, but it was nice to see somebody cared... I just wonder who?).