NEVER take a WR in round 1. I don't care about recent trends, most WRs don't break out for 3 years, blow up and then go sign somewhere else for big $ or cost you big $.
Let some other team develope a WR.
Defense, defense, defense.
The recent trends are what they are because of the proliferation of passing in colleges and pro style offenses, WR's are coming into the NFL more ready to contribute right away.
Not saying the Cowboys should take a WR in the 1st, but I think your 3 year rule is becoming outdated (Beckham, Evans, Hopkins)