From the Baseball Star Players Issue of 2012 through the Lunar New Year issue of 2016 most stamps were issued in no die cut sheets as another collectible from the USPS. All were valid postage. There were a number of ways of collecting: mint singles, mint pairs (block)s, mint panes or mint sheets.
Some of the panes are more valuable, esp. the 2015 Civil War and the 2013 Made in America. Of course, items such as a full no die cut panes of the Express Mail stamps are more valuable because of the high face value. For instance, a 2016 $22.25 Columbia River Gorge pane pf 3 has a face value of $222.50 (and a dealer selling price in excess of $700) while the full mini-sheet of 9 has a face value of $ 667.50.
My particular subset of collecting these is FDCs with the Digital Color Postmark. Extremely difficult to find. Here's my example of the last such issue on DCP FDC.