Of course it is. What ISN'T proper is to sit on the football for two quarters when you're only up by 10, which is what Cowher has a reputation for doing. Many a football game has been lost by him because of that sort of play calling; he has some sort of NFL record for consecutive victories when leading by 11, but if you drop it to 10, he's lost quite a few of them. The opponents have also been scary close to winning many, many more of them.
That being said, no shit when you're leading by 20. It's simply hilarious to watch Cowher call plays in the second half of a game they're leading by any score. When Cowher enters his Turtle Zone, the Steelers aren't coming out of the shell -- even when they've gone 3 and out the entire half. Teams like the Texans can't do shit against it, but on Halloween last year, the Patriots showed exactly how dangerous it can be by making a game of it in the fourth quarter.