I've had two or three sets of tires that were all but impossible to mount, including a pair of Vittorias that broke FIVE plastic levers (I eventually went to the grocery store bike aisle for a set of those old, cheap steel ones). I had a flat on the first ride and couldn't get them off to fix it, the only time in 30+ years of cycling that I've had to call for a ride home. I finally cut them off the rims and threw them away. This seems to be a fairly recent development, and I'm inclined blame fear of liability--the tire companies would be at risk if a tire came off the rim, even if it were underinflated or improperly installed, so they make them just a hair tight to reduce that possibility.
End of rant. As for stretching, it's standard with tubular tires, but I've never noticed that it makes a difference with clinchers. Both wire and Kevlar beads are pretty tough...on the other hand, a tire is about seven feet in diameter, and even a millimeter of stretch would make mounting easier, so maybe I haven't been paying attention.