You probably know that slicing meat against the grain makes sure it's never chewy or difficult to eat. America's Test Kitchen put that rule to the test, with some so-called "tough" and "tender" cuts of meat. Long story short, they found the way you cut can make any cut easier to chew, regardless of whether it's "tough" or "tender".
The full video tells the tale, but they point out that while most of us know that the internal cooked temperature of the meat and the cut itself can play a role in how tender or tough the meat is when it gets to the plate, how you slice it when you serve -- or while you cook -- also plays a big role. To test it, they took skirt steak (usually characterised as "tough" and critically important to carve properly) and porterhouse (usually "tender"), cooked them to 54°C.
Then they used a CT3 Texture Analyser to measure how much force was required to "bite" into the meat.
Sure enough, they found that for the skirt steak, when cut with the grain instead of against, it took four times as much force to "bite" about 5mm into the meat. That's a huge change when it comes to mouthfeel and chewiness and a great case to cut against the grain. Well, they actually found the same was true with the traditionally "tender" cut as well -- while skirt was definitely tougher when sliced with the grain than the porterhouse, when sliced against the grain, they were both about the same tenderness.
That doesn't mean you can run out and replace all your porterhouse steaks with well-cooked, well-carved skirt, but it does mean that "tough" cuts don't have to be -- if you serve them correctly.
Science: How to Slice Steak [Cook's Illustrated]